ND   Jacques Maritain Center : Psychology / by Michael Maher, S.J.

CHAPTER VI.

PERCEPTION OF THE MATERIAL WORLD:
CRITICAL SKETCH OF THE LEADING THEORIES OF EXTERNAL PERCEPTION.

Psychology and Philosophy of Perception. -- How do we perceive the External Material World? and: What are our grounds for believing in its real existence? These are the problems which have most harassed Philosophy since the days of Descartes. The two questions, the Nature of external perception and the Validity of our belief in a material universe, are most intimately bound up with each other. The worth of every theory of cognition must be estimated by the sufficiency of the account which it gives of the reality that is known. Accordingly, though only the question of the character of the process of apprehension is strictly psychological, while the validity of the act belongs to Epistemology{1} or Applied Logic, we shall find it very advantageous in the interests of our own science to trespass here a little on the domain of

another volume of the present series. This impossibility of separating the problems of the genesis and the truth of knowledge shows again the futility of all attempts at isolating Phenomenal Psychology from Rational Psychology and Philosophy proper.

Sceptical Theories. -- Let us begin with the more fundamental question: What are our grounds for believing in the existence of a Material World outside and independent of our thought? The answer given by certain philosophers is that there are no real grounds for this belief, and that it is an illusion, or, at any rate, an irrational prejudice. This is Scepticism. Now scepticism may be of either of two species: the one, absolute or universal, which denies or disputes the possibility of attaining certitude by any of our faculties, or in any department of knowledge; the other mitigated, limited, or partial scepticism, which, admitting certain truths as evident, and certain faculties as infallible sources of cognition, yet discredits some convictions of mankind generally deemed to be of vital importance. Against absolute scepticism argument is alike useless and impossible. Its advocate is in an impregnable position, because he puts himself outside the pale of discussion. Nothing can be done for such a man except to leave him alone. Of partial or mitigated sceptics there are many varieties, but our concern here is only with that class, commonly called Idealists, who deny the existence of an independent material world. Several of these philosophers will be refuted in detail in our historical sketch in the latter part of this chapter, and an exhaustive treatment of scepticism in general is to be found in the volume of this series on First Principles of Knowlcdge.{2} Accordingly, we will here limit ourselves to a brief enumeration of the arguments establishing the existence of an external material world.

Philosophical proof of Realism. -- (1) The reality of other minds is admitted, we believe, by every sect of idealists falling short of absolute scepticism. But our assurance of the existence of other minds is only an inference from changes in the bodies which they animate. Consequently we cannot deny the existence of the latter outside of our own consciousness and maintain the independent reality of the former. But if we admit the existence of other human bodies, clearly we cannot reject any part of the material unlverse. (2) The idealist cannot explain the course and development of his own mental life without implying the permanent extra-mental existence of his sense-organs and bodily frame. (3) The established relations between mental states and their neural conditions, and in fact all the chief truths of Physiology become unintelligible absurdities if the permanent existence of a material organism outside of our thought is denied. (4) Physical science in general assumes the existence of an independent material world, and the harmony of its teaching with later results verifies the assumption. (5) The mutual confirmation of our several senses, exhibited in experiences of sight, touch, and movement, similarly demonstrates the existence of a material universe outside of the mind. These faculties, which present to us the extensional character of physical objects in widely different terms of consciousness, nevertheless agree unanimously as regards the spatial relations of parts to parts. The diagonal, for instance, bears the same proportion to the sides of the square, whether the lengths of the lines be apprehended by visual, tactual, or motor sensations. Now this unanimity is perfectly accounted for if by our several faculties we perceive a material world which really embodies these spatial relations. But if there does not exist an extended reality outside of our consciousness this agreement in the testimony of different witnesses is inexplicable.

Psychology of External Perception. -- Theory of Mediate Perception. -- The arguments just given will be more fully developed in the historical sketch at the end of this chapter, but their mere summary statement is sufficient to establish the existence of an extended material world of which our body forms part. The psychological question now emerges: How do we perceive or know this outer universe? Answers to this question, in spite of many important minor differences, may for the present be reduced to two. On the one side the majority of non-Catholic philosophers since the time of Descartes assume that the unextended mind cannot have an immediate apprehension of extended reality in any form. It can directly know only its own states. Consequently the chief effort of modern speculation has been, either, assuming the existence of a Material World, to explain how from a knowledge of purely subjective feelings the mind can attain to the cognition of such an extra-mental reality, or, rejecting the existence of this latter, to account for, the universal illusion.

Philosophers believing in some sort of an independent Material World, who maintain that the mind can only attain to a knowledge of such a world mediately as an inference from the ideas, or subjective representations, of which alone we are immediately cognizant, have been styled Representationalists or advocates of Mediate Perception. They have also been called Hypothetical Realists, Hypothetical Dualists, or Cosmothetic Idealists, since they look on the external universe as a necessary hypothesis to account for the ideas of which we have an immediate perception. All these authors err in the one common but groundless assumption that the human mind can immediately know nothing but its own unextended states. Starting from this false hypothesis, their theories give no adequate account of our knowledge of extension, and logically lead to subjective Idealism. We will expose some of their chief defects presently in our Historical Sketch.

Immediate Perception. -- In complete opposition to Representationalism are to be found Aristotle, all the leading scholastics,{3} mediaeval and modern, and in this country during the past hundred years, Reid, Stewart, and Hamilton. At the present day Drs. Martineau, Mivart, M'Cosh, and Porter, are amongst the best known English-speaking representatives of the same line of thought. All these philosophers, notwithstanding sundry lesser points of disagreement, hold that man, at all events in some cognitive acts, immediately apprehends extended material reality. They teach that knowledge is not limited to the perception of mental states, or to the discernment of the relations between ideas. There are outside and independent of the world of thought real things; and we can, these writers agree in common with the universal conviction of mankind, cognize at least some of them. This theory has been named by Hamilton the doctrine of Immediate or Presentative Perception, because it asserts that some objects of knowledge can be immediately present to the knowing subject. Its supporters have also been styled Natural Realists, and Natural Dualists, because they maintain the existence of extended material reality standing in opposition to the immaterial mind to be a primitive deliverance of our percipient faculties.

We hold the true doctrine to be that of Immediate or Presentative Perception. My present knowledge of an extended material universe independent of my mind is inexplicable unless at least in some of my percipient acts there is contained an immediate apprehension of extension; and this apprehension necessarily reveals a duality or opposition between the simple subject of consciousness and the objective material reality. The growth and development of our several percipient faculties will be described in detail in our next chapter, so that it will be our duty here merely to expound accurately what we consider to be the general philosophical theory of Presentative Perception.

Ambiguity of Terms. -- We must begin by clearing up certain confused notions which have often obscured and disfigured the treatment of the problem, not only on the part of our opponents, but even in the hands of some able and vigorous defenders of Immediate Perception, especially among the Scotch school. The exact meaning to be assigned to the terms, Ego and Non-Ego, Self and Not-Self, Mind and External World, in this controversy is of the very first importance; or rather the vital point is that whatever definite significations are attached to them be adhered to throughout.

Ego and Mind. -- Now in the first place by the term Ego is to be understood during the present discussion the entire person, the whole man made up of body and soul. The Non-Ego is, therefore, whatever is not part of my person. In strictness it includes God and the universe of pure spirits; but as the reality of immaterial beings does not enter into our present controversy, we may define the Non-Ego as, the Material Universe distinct from my own animated organism. Self and Not-Self are to be considered as synonymous with Ego and Non-Ego. The terms, Mind and External, or better, Extra-Mental World, must be carefully distinguished from the former pairs of words. Abstracting from all questions as to the substance of the soul, by Mind we here understand the unextended conscious subject, the unity of my psychical existence, viewed apart from my body. By the External or Extra-Mental World, is meant all material reality, including both my own body and the extra-organic universe. Mind is thus narrower than Self or Ego, and External World is wider than Not-Self or Non-Ego.

Man not a Pure Spirit. -- In the second place we must make clear our starting-point. Some representationalists often argue as if the mind were de facto completely separated from the body, or at any rate standing out of all relations to the corporeal frame. What would be the nature of perception in such a situation we do not pretend to determine: it is not the problem of Human Psychology. We take man as he is; one being made up of mind and body, endowed with sensuous as well as intellectual faculties, and possessed of a variety of extended sense-organs, the natural instruments by which he acquires knowledge, not only of the surrounding world, but of his own body.{4} Two Questions. -- Now in the problem of the Perception of the Material Universe, two points connected with the ambiguous terms just defined, and consequently almost invariably confounded, have to be kept apart. They are, in fact, two distinct questions -- the one, my apprehension of extension and extra-mental reality in any form, the other, my cognition of the Non-Ego or Extra-Organic portion of the material world. To begin with the first: we hold it to be certain that at all events in the case of its own organism the Ego has an immediate perception of extension. In sensations of sight and pressure there is directly revealed space of two dimensions. Whether the cause of the sensation is externalized, projected beyond the surface of the extended organism, or not, the conscious state aroused immediately presents extension. The proof of this lies in the fact that if extension were not so given the perceptions and conceptions of space of which in mature life we are indubitably possessed could never have been generated. If the mind knew only its own simpIe subjective modifications, our present cognition of material objects would be impossible. No aggregation, composition, or fusion of mental states which individually do not present any element of extension, could produce the notion of extension. If some of our senses have directly revealed space to us, the representations of material objects which we form can be accounted for; if none of them had done so, these representations could never have arisen. This argument will be more fully developed when we come to criticize in detail the theories advanced to explain the genesis of an external world of three dimensions out of simple conscious states. Immediate perception of Extension. -- Next comes the question: Do any of our percipient acts immediately make known to us the existence of a reality other than ourselves? It is here precision and consistency in the use of the terms Ego, External World, and the rest, become vitally important for clearness of thought in the present discussion. We have said that in certain percipient acts, more particularly in those of sight and touch, there is given an immediate presentation of extension: Of what is this extension apprehended to be an attribute? To what is it cognized to belong? In mature life, undoubtedly, we perceive in an apparently instantaneous flash of cognition that the object against which we press is a soft velvet cushions that what we see is a red-brick house at the far side of a river. But this does not settle the question, for in these acts there demonstrably are involved complex processes of inference or association of ideas. Taking, however, the sensations of vision and pressure in their simplest form, do they immediately give, in addition to the perception of extension, a knowledge of material reality as distinct from the percipient agent? The solution of this question will be found in reverting to our distinctions. In the simplest percipient act which directly reveals extension there is given an immediate apprehension of "otherness," at least in the sense of the extra-mental. Extension, whether it pertains to our own sense-organs, or to objects outside of our body, is at all events not an attribute of simple mental modifications. It is opposed to the subjective conscious act. Consequently, although in the earlier stages of life such distinctions may not be explicitly realized, there is given in the immediate presentation of extension -- whether this extension be referred to the Ego, to the Non-Ego, or not determinately to either -- an immediate apprehension of what is not the Mind. There is thus an ultimate duality in our consciousness at least in this signification that some of our faculties are capable of immediately apprehending extension, and extension thus apprehended necessarily stands opposed to the unextended mind.

Perception of extra-organic Objects. -- But is Duality immediately given in the wider sense? Does the percipient act not only directly manifest to me an extended phenomenon irreducibly opposed to the simplicity of the purely subjective state, but does it also immediately reveal this extended phenomenon as other than my Ego, other than my Self in the sense of my whole being, body and soul? or is my knowledge of the existence of a Non-Ego in the strict sense -- of a material world outside of my own body -- is this cognition of a more complex, mediate, and possible inferential character? This is certainly a more disputable point. The majority of Natural Realists seem at times to imply that the Non-Ego in the sense of Extra-organic material reality is originally presented as extended, distinct from, and opposed to my whole bodily self; but the distinction between the two uses of the term Ego -- as including and as excluding the organism -- is on such occasions rarely kept clearly in view. The second, or qualified form of Natural Dualism, would maintain that, whereas extension, and therefore objective reality, standing in opposition to the mind, is originally immediately given in sensations of my own organism, yet cognition of material reality as external to my organism is a result of analysis, comparison, and inference. This view, in fact, holds that our perception of the extra-organic universe, although in the developed intelligence so easy and rapid, is nevertheless a complex process.

It does not appear to us that this second form of the doctrine of Presentative Perception is always realized with sufficient distinctness. The Non-Ego may, indeed, be originally and immediately presented in some of the infant's first percipient acts as extrinsic to its organism. But this is not necessary to account for our later knowledge. Fortunately, however, this second stage of the problem of Perception is of little or no philosophical importance; and at any rate the line of demarcation between inference and immediate judgment is not very well defined. It is essential that extension, and consequently, a reality opposed to the unextended subject of consciousness, be directly presented, but granted such an immediate perception even limited to the spatial character of my own material organism, our knowledge of the rest of the universe would be easily developed.{5} In the next chapter we shall describe this process of development. Before doing so, however, we shall insert a historical retrospect.

HISTORICAL SKETCH OF MODERN THEORIES OF EXTERNAL PERCEPTION.

The question of External Perception has played such a large part in modern philosophical speculation that we deem it expedient to attempt a brief sketch of the subject. And we do this all the more willingly because experience has assured us that here, as often elsewhere, the most convincing proof of the true doctrine is to be found in a careful examination of the history of counter-hypotheses.

Descartes (1596-1650), whose philosophical speculations start from the dictum that I have an immediate and infallible knowledge of my own thought and of nothing more, may be justly considered the author of the problem of the bridge from the mind to the material world. It is to Locke (1632-1704), however, that the various forms of British scepticism, together with the idealism of Kant, are to be traced. Knowledge, Locke repeatedly maintains, consists in the perception of agreement or difference between our ideas. We thus immediately apprehend, not an external reality, but our own mental states. Nevertheless, Locke holds that a material world does exist outside of the mind. He is thus a Hypothetical Dualist. We only know psychical representations but we posit as their cause a physical universe.

Bishop Berkeley (1685-1753) soon made manifest the inconsistencies of Locke's teaching. Berkeley is celebrated chiefly for two contributions to the history of Philosophy, his system of Phenomenalistic Idealism and the Theory of Vision known by his name. The essence of the latter is contained in the two tenets that the eye of itself can perceive neither (a) distance, nor (b) surface extension. Visual sensations had originally as little reference to space as sounds or tastes. By experience and association, the sensations of the eye grow to be symbols of tactual and motor sensations which constitute our knowledge of solid bodies and of space of three dimensions. From this account of the psychology of perception the transition to his metaphysical theory of the nature of the External World is easy. Locke's groundless assumption that we can immediately perceive nothing but our own mental states, is accepted without question. All objects of knowledge are held to be reducible to ideas of the senses (sensations), internal feelings such as emotions, and acts of the imagination. Accordingly, we may not assert the existence of an independent extra-mental world. We can know or perceive only what is in the mind. The esse of every knowable object is percipi. If material substances existed beyond consciousness, they could in no way be like our ideas, and cognition of such things by ideas would be impossible. Moreover, matter could not act upon an unextended spirit. Therefore the hypothesis of an inert corporeal world which has existed for a time unperceived must be abandoned. Still, Berkeley vigorously. asserted that his theory is in complete harmony with the belief of mankind. The table, chair, or fire, which I perceive, he does not deny to exist; but, adhering to Locke's assumption, he calls whatever is apprehended an idea and going still further he repudiates the hypothetical material cause supposed by his master to have awakened these ideas. But whence then do these ideas come, and what happens when I cease to perceive them? Berkeley replies that God, and He alone, is the cause of my ideas. By the Divine agency, and not by any hidden inconceivable material substance, the permanence, regularity, and orderliness of the ideas are sustained. When I no longer think of ideas (material objects) they still endure in the Divine mind, and may be apprehended by other men. In Berkeley's system, then, there are held to exist minds or spiritual substances, ideas, and the Divine spirit.{6}

David Hume (1711-1776), similarly starting from Locke's principles, pushed Berkeley's Idealism to the most absolute scepticism. All cognitions, or all objects of cognition -- for with these writers the terms are interchangeable -- are reducible to impressions (sensations) and ideas, fainter copies of the former. To explain our belief in a permanent external reality, as well as to account for our other fundamental convictions, Hume appeals to the laws of the Association of Ideas. Through "custom" by the reiterated occurrence of various impressions we grow to believe in the enduring existence of material things when unperceived. Such belief is, however, an illusion; we only know the transient mental impressions. There is no such thing anywhere as an abiding substance, the substratum of changing qualities or accidents. We have no "impression" of it, therefore it does not exist. Berkeley got thus far as regards the notion of material substance; but Hume logically shows that by the same reasoning the idea of a spiritual substance, of a permanent mind amid changing states of consciousness is equally fictitious and unreal. The mind, just as well as the material world, is nothing more than a cluster of transitory impressions. The persuasion that nothing can begin to exist without a cause is also due to association. No single experience could give us the idea of causation; but the frequent repetition of two successive impressions so welds them together in our minds that we are deluded into the belief of some mysterious causal knot binding them, while there is really no connexion but that of succession. This illusory belief in particular instances of causality is afterwards gradually widened into the universal law, that every being which begins to exist presupposes a cause.

We have here all the essentials of later associationism. The substantial souls, retained by Berkeley, follow the material world of Locke, and the Divine Spirit also becomes a useless and inconceivable hypothesis. Hume, too, possessed the merit of realizing clearly and frankly admitting, what subsequent disciples of sensism either fail to see, or attempt to ignore, that the groundwork of physical science, and the certainty and exactness of mathematics are fatally destroyed by consistently following out the assumptions of the school. The conclusions of the Scotch sceptic thus constitute a complete reductio ad absurdum of Locke's principles.

J. Stuart Mill and Dr. Bain. -- The chief modifications introduced into the general theory by more recent sensationalists, are the final dismissal of Berkeley's hypothesis of the Divine action, the greater importance assigned to the muscular sense, and a more elaborate attempt to harmonize the new conception of the external world with ordinary beliefs. However, the arguments are in the main similar in kind to those urged by the earlier advocates. Thus, it is asserted, that a world existing independently of the mind is inconceivable. "To perceive is an act of the mind. . . . To perceive a tree is a mental act; the tree is known as perceived and not in any other way. There is no such thing known as a tree wholly detached from perception, and we can only speak of what we know." Consequently, the hypothesis of an external world existing when unperceived is absurd. "The prevailing doctrine is that a tree is something in itself apart from all perception; that by its luminous emanations it impresses our minds, and is then perceived, the perception being the effect of an unperceived tree the cause. But the tree is known only through perception; what it may be anterior to or independent of perception we cannot tell; we can think of it as perceived but not as unperceived. There is a manifest contradiction in the supposition, that we are required at the same moment to perceive the thing and not to perceive it."{7}

The "Psychological" or Empiricist doctrine of our belief in matter. -- The chief strength, however, of the theory lies in the asserted sufficiency of the account which it professes to give of the material world apprehended by us. Assuming as self-evident the axiom that we can know only our own ideas, the external universe, it is alleged, really means to us nothing more than certain sensations plus possibilities{8} of other sensations. The most objective and real attributes of material things are in common belief their extension and impenetrability. Nevertheless, these properties, it is asserted, are ultimately reducible to groups of muscular feelings possible and actual. "The perception of matter, or the object consciousness, is connected with the putting forth of muscular energy as opposed to passive feelings. . . . Our object consciousness further consists of the uniform connection of Definite feelings with Definite energies. The effect that we call the interior of a room is in the final analysis a regular series of feelings of sense related to definite muscular energies. A movement one pace forward makes a distinct and definite change in the ocular impressions; a step backwards exactly restores the previous impression. . . . All our so-called sensations are in this way related to movements. . . . On the other hand, what in opposition to sensations we call the flow of ideas -- the truly mental or subjective life -- has no connection with our movements. We may remain still and think of the different views of a room, of a street, of a prospect in any order."{9}

The apparently independent world of every-day experience has not suddenly manifested itself to us after the manner of a transitory hallucination. It is a gradual growth, and it is in tracing the supposed genesis of this illusory belief that Mill best exhibited his psychological and metaphysical ingenuity. Starting with the postulates of expectation, the occurrence of impressions, and the laws of mental association, he professes satisfactorily to explain all our present convictions. We experience, he says, various sensations, such as those of colour, sound, and touch. After they have passed away we conceive them as possible. These feelings usually occur in groups, thus the consciousness of yellow is found in combination with certain sensations of contact, of smell, and of taste, which go to make up our perception of an orange. Similarly, visual feelings precede the tactual sensations which we have come in course of time to call the table. By association the groups of states become so knotted together that one of them by itself is able to awaken in idea the rest, and to suggest them to us as possible experiences. A material object is, in fact, to us at any time one or two actual feelings with the belief in a suite of others as possible. The actual impressions are transient; the possibilities are permanent.

In addition to the feature of permanence and fixity among these groups of possible impressions there is the constant and regular order which we observe among them. By association this gives rise to the notions of causation, power, and activity; and we gradually come, on account of their permanent character, to look upon the groups of possible sensations as the cause of the actual feelings. Moreover, finding changes to take place among the possibilities of our impressions independently of our consciousness, we are led by abstraction to erect these possibilities into an entirely independent material world. This operation is completed by the discovery that other human beings have an experience similar to our own, and ground their conduct on the same permanent possibilities as ourselves. Besides the apparent permanence and independence of the material world, its most striking contrast with our sensations lies in its extension and impenetrability. The latter property, however, is merely the feeling of muscular action impeded. Space is similarly an abstraction from motor feelings. Muscular sensations differing in duration give us the consciousness of linear extension inasmuch as this is measured by a sweep of a limb moved muscles. . . . The discrimination of length in any one direction includes extension in any direction." Not only is the idea of space derived from non-spatial feelings successive in time but this mode, "in which we become aware of extension is affirmed by the psychologists in question to be extension." "We have no reason for believing that space or extension in itself is anything different from that by which we recognize it."{10} The synchronous character of space receives its completion from sight, which presents to us simultaneously a vast number of visual impressions associated with possibilities of motor and tactual feelings. Such is the empiricist theory of our belief in a material world.

Criticism. -- Phenomenal Idealism as thus advocated has been attacked from many different points of view, but we can here afford space for only a few of the leading difficulties which seem to us absolutely fatal to the hypothesis. (1) In the first place, as we have already indicated, Idealism is incompatible not only with vulgar prejudices, but with the best established truths of science. Astronomy, Geology, Physical Optics, and the rest of the physical sciences, are inseparably bound up with the assumption that matter which is neither a sensation nor an imaginary Possibility of a sensation exists apart from observation. They teach that real, actual, material bodies, of three dimensions, not only exist, but act upon each other according to known laws whilst no human mind is contemplating them. Possibilities enjoying no existence beyond consciousness could not attract each other with a force varying inversely as the square of the distance; they could not pass from green forests into coal beds, nor could they refract or interfere with other phenomena so as to determine the character of visual sensations independently of our wills. How, for instance, is the double discovery of the planet Neptune from the simultaneous but independent calculations by Adams, and Leverrier, to be explained, if there are not in the universe besides human minds extended agents which retain and exert their influence when unthought of by any created intelligence.

(2) This irreconcilability between physical science and phenomenal Idealism results in a very noteworthy case of felo de se in the hands of Dr. Bain. He commences his works on Empirical Psychology with an elaborate account of the brain, the nervous system, and the various sense-organs. Later on in the same volumes be resolves the material world, including, we presume, the aforesaid objects, into a collection of mental states. Finally, in his book on Mind and Body, he resolves the mind, that is, the total series of conscious states, into subjective aspects or phases of neural currents. Now obviously there is at least one absurdity here. What is the exact meaning of the statement that a mental state is but the subjective aspect of a nervous process, which is itself but a group of sensations? At one time the mind is alleged to be a function of the brain, and elsewhere the brain, with the rest of the physical universe, is analyzed into a plexus of muscular feelings incapable of existing beyond consciousness. These two mutually destructive tenets, Phenomenal Idealism and Physical Materialism, are the logical outcome of the sensist theory of cognition; but unfortunately disciples of that school do not usually reason out on both sides the consequences of their assumptions with the clearness and courage of Dr. Bain. The only subject for regret is that the latter writer neither attempts to reconcile the two repugnant theses, nor frankly avows that they form a reductio ad absurdum of his theory.{11}

(3) Again, the primary assumption on which all phenomenalistic theories since the days of Locke have been based is false. That we can only know our own mental states, that we cannot apprehend material reality as affecting us is neither an a priori nor a self-evident truth, and still less can it be established from experience. The fact that we are unable to imagine how matter can act upon mind, or how mind can become immediately cognizant of something other than itself, is no objection against the clear testimony of consciousness, as manifested after the most careful introspection, that the mind does immediately perceive something other than itself acting upon it. Moreover, from this first illegitimate assumption flows the second error, that extension is identical with that by which it is measured. The velocity of a moving locomotive or of a flying swallow is not the same thing as its force. Now, our knowledge of extension may receive accurate definition and determination, mainly by means of the muscular sensations, and yet what we call the extension of objects may be not only something different from these sensations, but it may also be immediately apprehended in a less defined manner through some other senses.

(4) Further, we must deny in toto that sensations, muscular or any other, viewed in themselves as purely subjective, nonspatial feelings, could ever by any process of addition or transformation be worked up into an apparently extra-mental world. It is only by the surreptitious introduction of extended elements that an extended product can be effected; and the great use made of the muscular sensations in the empiricist theory is due to the fact that the illicit transition from the asserted originally subjective signification of motor sensations to the objective meaning implied in ordinary beliefs is liable to escape notice. If these feelings are steadily remembered to be simple states of consciousness varying only in duration and intensity, it will be seen that they cannot, any more than sensations of sound or smell, "consolidate" into extended objects. Duration -- serial length in time -- belongs to all sensations, yet many of these afford no knowledge of space, much less constitute it. Sensations may also vary in intensity without evoking the notion of velocity; this latter cognition, in fact, presupposes the idea of space.

In all associationalist accounts of the genesis of our knowledge of an external world there is a continual equivocation between strictly mental existence and that which is intra-organic but not purely mental; between the signification of the terms describing the organism legitimate on their principles and the alleged erroneous meanings which these words convey to the vulgar mind. Notwithstanding all lofty disclaimers to the contrary, sensationalists when tracing the gradual manufacture of the material universe out of simple states of consciousness, really do assume the existence of an extended organism, as known from the first. When Mr. Bain, or Mr. Spencer, describes how muscular feelings, varying in duration and velocity, give rise to the belief in extended space, the explanation{2} seems plausible because the reader almost inevitably passes from the subjective interpretation, which is all that is lawful to the writer, to the objective realistic meaning embodied in common language. The phrases, "range of an arm," "sweep of a limb," and the like, employed by associationists in expounding the supposed origin of the notion of extension, necessarily suggest to the mind real extended objects known as such, and so conveniently hide the true difficulty. Commencing with a knowledge of our own body as extended, the development of our conviction of an independent material world might, perhaps, even on sensationist lines, proceed tolerably enough; but if our body and the rest of space are nothing more than sensations, and if the mind can only apprehend its own subjective feelings, then the first step is impossible. Successive muscular or tactual feelings in the interpretation of these sensations permissible to Mr. Spencer or Mill can no more account for the present appearance of extended objects than experiences of sound, of smell, or of toothache.

(5) The argument from the existence of other minds to which we have before alluded may also be here urged with peculiar force against Mill and Dr. Bain. Both of these writers lay stress upon the value of the testimony of other minds in establishing our belief in an independent world. Our knowledge, however, of other minds than our own is only gained by an inference from changes in certain portions of the physical world, assumed to have a real existence beyond our consciousness. Now if the chief premiss is invalidated, if it is demonstrated that we have, and can have no knowledge of anything external to our consciousness, that the seemingly independent human organisms around us are only modifications of our own mind, clusters of our muscular feelings actual and ideal, then assuredly it is an unworthy superstition to continue to put faith in the external existence of other minds, and still more ridiculous and absurd to invoke their testimony as a leading agency in the generation of our belief in the material world, including of course the bodies from which their existence is inferred.

(6) There remains another fundamental difficulty which goes to the very root of the sensationist philosophy. This genesis of space out of time necessarily implies, at all events, the existence of a permanent mind. Under the pressure of Dr. W. Ward's severe criticism, Mill was obliged in addition to his other assumptions to "postulate" memory. A mere succession of disonnected feelings could never give rise to the notion of time, and still less could the possibilities of such successive sensations be condensed by themselves into the simuItaneity of space. But memory is precisely what the doctrine which reduces the mind to a series of feelings has no right to postulate. An abiding subject permanent among our changing states is an essential requisite for the existence of memory. If, however, the notion of time is impossible to the sensationalist, a fortiori is that of space.

Emanuel Kant (1724-1804). -- A theory of perception equalIy erroneous with that of Hume's school, but starting from an almost diametrically opposite conception of the nature of the mind and of cognition, is that of Kant. Instead of explaining all mental products as complex results arising out of the aggregation, association, and coalescence of sensations passively received, Kant holds the mind to be endowed from the beginning with certain a priori or innate subjective "forms," by which all its experience is actively moulded or shaped. Among the most important of these are the two "intuitions" of Space and Time. The first is imposed on the acts of external, the second on those of internal sensibility. The sensations of our external senses are non-spatial in themselves, and they are awakened by a non-spatial cause. It is the subjective co-efficient that shapes the mental act so as to give rise to the perception by which we seem to apprehend extended objects outside of the mind. Similarly our mental states are presented to us by the internal sense -- inner consciousness -- as occurring in time. This, too, is an illusion due to a purely subjective factor in cognition. We have no reason for supposing that these states are not timeless in themselves. We can only know phenomena, or the appearances of things as shaped and coloured, by these subjective conditions; to noumena, or things-in-themselves, we can never penetrate. Still the existence of a noumenon beyond consciousness Kant maintains as requisite to account for our cognitive acts. He is thus a Hypothetical Dualist, denying an immediate apprehension of an external reality, but asserting its existence as a necessary supposition.

Criticism. -- Deferring to a later chapter the examination of Kant's system as a whole, we may here indicate a few of the objections suggested against his treatment of the subject matter at present under discussion. In the first place, it has been urged that Kant's attempted proof of the existence of a priori ingredients in all our knowledge is invalid. (a) "Space," he argues, "is not a conception which has been derived from outward experiences. For in order that certain sensations may relate to something without me . . . and that I may represent them not merely without and near to each other but also in separate places, the representation of space must exist as a foundation. Consequently, the representation of space cannot be borrowed from external phenomena through experience, but, on the contrary, this external experience is only possible through the said antecedent representation."{12} Space is, therefore, a purely subjective a priori form, inherent in The constitution of the mind, and imposed on the material element given in sensation.

This method of reasoning was employed by Plato to show That all knowledge is really innate. It sins by proving too much. If it were true that we could not apprehend an object as extended unless we had a previous representation of extension, then it would seem to follow that we could never cognize a taste, sound, or smell, unless we had antecedently a similar cognition of the nature of the taste, sound, or smell. If there are in existence extended material bodies, and if we are endowed with the faculties of touch and sight, there is no reason why we should not immediately perceive the spatial qualities of these bodies when they act upon our senses. The perception may of course be at first vague, but frequent experience can perfect it.{13}

(b.) "We never can imagine or make a representation to ourselves of the non-existence of space, though we may easily enough think that no objects are found in it. It must, therefore, be considered as the condition of the possibility of phenomena, and by no means as a determination dependent on them, and is a representation a priori, which necessarily supplies the basis for external phenomena." (p. 25.) This difficulty is solved by distinguishing between actual or real space, and possible or ideal space. The former is identical with the voluminal distance or interval enclosed by the surface-limits of the entire collection of created material objects, the latter is simply the possibility of extended objects. Now, although all material things were annihilated, the possibility of their existence, and therefore possible space, would remain. Consequently, having once apprehended the extension of existing bodies, we can never think them to be impossible, although we may abstract from their existence. The conception of ideal space, or the possibility of material bodies, is thus indestructible, not because it is merely a condition of thought, but because it is a condition of corporeal being.

(c) "Space is no discursive or, as we say, general conception of the relations of things, but a pure intuition. For, in the first place, we can only represent to ourselves one space, and when we talk of divers spaces we mean only parts of one and the same space. Moreover, these parts cannot antecede this one all-embracing space, as the component parts from whicb the aggregate can be made up, but can be cogitated only as existing in it." Again, (d) "Space is represented as an infinite given quantity." To these arguments we may again reply that a general conception of the relations of material things, or an abstract notion of the possibility of extended objects, may be formed from many perceptions of different parts of space. The fact that such an idea of possible space represents the latter as infinite, or rather indefinite, one, and all embracing, in no way proves that this representation is given a priori.

Kant further holds that the necessity and universality which characterize geometrical judgments establish the subjective origin of our cognition of space. This must be denied. Objects without the mind may have certain modes or relations of a contingent and others of a necessary nature. But if such were the case there can be no reason why the mind should be incapable of apprehending both with equal truth. The explanation put forward by Natural Realism is that there are certain essential and certain other accidental conditions of material being, and that these are reflected by necessary and contingent features in our thought. This is a simple and adequate account of the problem without the gratuitous assumption of innate forms.{14}

Still even were it true that our knowledge of external objects in no way represented them, the doctrine of Kant that our apparent cognition of our own mental states as they are in-themselves is deceptive, would be erroneous. In this region, at least, the distinction between phenomenal knowledge and noumenal existence is utterly invalid. A conscious state cannot have any existence-in-itself apart from what it is apprehended to be. Its esse is percipi. Since, then, mental states are as they are apprehended, and since they are apprehended as successive, they must form a real succession in-themselves. They cannot be timeless as they are non-spatial. But if so Kant's "form of the internal sense" -- the intuition of time -- is extinguished. According to him time, like space, is merely a subjective condition of our internal consciousness imposed on realities timeless in themselves. As, however, there is a real succession in our ideas, there is a true correlate to the notion of time. A sequence of changes being once admitted in our conscious states, an analogous succession of alterations cannot be denied to the external reality which acts upon us, and so we are justified in maintaining the objective validity of the notion. The whole growth and evolution of each man's mental life, and its connexion with the development of his organic existence, affords the most cogent conceivable evidence of the real truth of the conception of time.

Further, the arguments already put forward against Phenomenal Idealism show that neither space nor time can be a purely subjective form. Physics and astronomy, for instance, are irreconcilable with such a view. Thus, the latter science by a series of elaborate deductions from (a) abstract geometrical theorems dealing with the properties of pure space, and (b) dynamical laws describing the action of unperceived forces in an orderly manner in time, foretells a transit of Venus a century hence, and the result verifies the assumptions. Now the introduction of the second element is peculiarly incompatible with the alleged subjective nature of space. A consistent system of pure geometry might perhaps be worked out in such an a priori space, but there would be no reason why its theorems should exactly apply to the operations of extra-mental non-spatial agents. Accordingly, the orderliness of the universal force of gravitation, which varies inversely as the square of the distance, and produces regular movements in certain intervals of time, establishes agreement between the supposed mental forms and the reality beyond consciousness.{15} The physicist also teaches us at the external causes of our sensations of colour and sound are vibratory movements of ether (in extra-mental space) occurring in succession (in extra-mental time). He further informs us that the quality of the sensation is determined by the size and rapidity of these waves. Now this teaching is irreconcilable with the view that the supposed space and time are merely subjective forms of outer and inner sensibility. It implies that the so called noumena, the extra-mental causes of our sensations of colour, occupy a real space of three dimensions, antecedent to and independent of the observation of the percipient mind.{16}

In addition to these objections a number of other defects in Kant's system have been exposed. He assumes without investigation the false representationalist theory in vogue since the times of Descartes and Locke, teaching that we have no immediate knowledge of things affecting us, but only of our own mental states. He illogically postulates an external noumenal world as the cause of our conscious states, whereas he has no ground for asserting its existence, especially since he teaches that causality is another deceptive intellectual form with no objective value. Finally, he is confused and inconsistent in expounding the nature of the supposed a priori forms, frequently appearing to conceive them as complete representations, ready made from the start and fitted with perfect accuracy on to the first act of perception, whilst at other times be seems to look on them as slowly and gradually realized with extended experience.

Mr. Herbert Spencer, starting from the same assumptions as Hume and Mill, nevertheless rejects Idealism, substituting in its place a species of Hypothetical Dualism which he calls Transfigured Realism. With him, as with them, we can know nothing but our own feelings; yet he affirms that there is outside of the mind an Unknowable Reality, the objective cause of our sensations. But beyond the fact that such a noumenon exists, we can assert nothing of it. "What we are conscious of as properties of matter, even down to weight and resistance, are but subjective affections produced by objective agencies, which are unknown and unknowable."{17} His defence of this theory is based on an analysis of our mental operations akin to that of the older Associationists, supplemented by an argument against the Idealism of these writers extending over some nineteen chapters. The chief proofs which he urges against Idealism are these: (1) Priority. -- In the history of the race, as well as in the history of every mind, "Realism is the primary conception," and Idealism is merely derived from and subsequent to the former. (2) Simplicity. -- The chain of reasoning establishing Realism is simpler and shorter than that proving Idealism. The latter, too, depends on the former. (3) Distinctness. -- The doctrine of Realism is presented in distinct and vivid terms, whilst Idealism can be apprehended only in a vague and obscure manner. (4) Realism is established by the criterion of the Universal Postulate. We must accept as true what we are obliged to think, and we cannot think away the existence of the objects which we perceive.

We can only touch on one or two points of this theory here. In the first place, though Mr. Spencer's arguments are undoubtedly valid against the idealist, they are not less efficacious against his own system. All the proofs from simplicity, priority, the application of the Universal Postulate, and the rest, tell equally in favour of Natural Realism against Transfigured Realism as expounded by himself. In the second place, Mr. Spencer's Transfigured Realism is little, if at all, fitter to meet the demands of science than Kant's non-spatial noumena or Mill's possibilities of sensation. Accordingly, for disproof of the new hypotheses, we refer the reader back to the arguments we have been just expounding. Physical science asserts much about the internal relations of the extra-mental causes of our sensations, which implies the existence of a real time, and of a space of three dimensions apart from our consciousness, yet truly mirrored by the features of that consciousness. Mr. Spencer's own statement, too, that there are variations in the modes of the asserted Unknowable corresponding to our consciousness of changes in space and time, abandons his most important tenet that we can know nothing about the Unknowable except its existence. The same difficulty which proved fatal to the theories of Mill and Kant tell equally against Mr. Spencer. Neither the assumptions nor conclusions of Physical Science can be confined within the territory of phenomena. The notions of "energy" and "force" lying at the root of mechanics and physics, and the laws of their action which science professes to expound, imply that the mind has a real valid knowledge of the supposed noumenal or unknowable causes of our sensations. Finally, Mr. Spencer's reduction of the material world, which we appear to perceive, into groups of feelings is based, like that of Hume and Dr. Bain, on the false assertion that we cannot have an immediate knowledge of external reality.

Probably the best exposition in English of the above line of argument, based on the conflict between Empiricism and Physical Science, is that to be found in Mr. Arthur Balfour's Defence of Philosophic Doubt, chapters ix. and xii. Viewed as an argumentum ad hominem against the school of Mill and Spencer, the reasoning there is perfectly valid, and seemingly unanswerable, though in other respects some of the sceptical conclusions appear to us to be overdrawn.

Readings -- The First Principles of Knowledge, by John Rickahy, Pt. II. c. ii.; Dr. Mivart, Nature and Thought, c. iii.; On Truth, cc. vii.-xi.; Dr. Martineau, A Study of Religion, Vol. I. pp. 192-214; Hamilton, Metaphysics, Lect. xxv.-xxviii.; Professor Veitch's Hamilton, cc. v-vii; Dr. M'Cosh, Exam, of Mill, cc. 6, 7; Ueberweg, Logic, §§ 37-44; R. Jardine, The Elements of the Psychology of Cognition, pp. 47-58, 125-148. The whole subject is very ably handled by A. Farges in L'Objectivité de la Perception des Sens externes et les Théories Modernes (Paris, 1891). See also J. Mark Baldwin, The Senses and the Intellect, pp. 134-138. The ablest treatise however in English on this subject is Professor T. Case's Physical Realism (Longmans).


{1} Epistemology is that branch of Philosophy which, whether it be allotted to Applied Logic, Rational Psychology, or Metaphysics, investigates the truth or validity of knowledge in general. It is separated by modern psychologists from their science, which, according to them, has to deal only with the genesis and growts of knowledge. {2} Cf. Pt. I. c. viii. and Pt. II. c. ii.

{3} See pp. 52, 54.

{4} It may be well to remind the student here that this assumption of an extended human body does not involve us in any petitio principii. We are not now proving the existence of a material -- that we have done some pages back -- but we are explaining how man perceives this world.

{5} Thus Hamilton justly observes: "It is sufficient to establish the simple fact, that we are competent, as consciousness assures us, immediately to apprehend the Non-Ego in certain limited relations; and it is of no consequence whatever, either to our certainty of the reality of the material world, or to our ultimate knowledge of its properties, whether by this primary apprehension we lay hold, in the first instance, on a larger or a lesser portion of its contents." (On Reid, p. 814.)

{6} Berkeley's theory may be objected to on various grounds, such as his equivocal use of the terms idea and conceive, and his unquestioning acceptance of Locke's assumption, but we have never seen any experiential argument which, strictly speaking, disproves the hypothesis of hyperphysical Idealism. God, without the intervention of a material world, could potentiâ absolutâ immediately produce in men's minds states like to those which they experience in the present order. The only demonstrative argument against the Theistic Immaterialist is, that such a hypothesis is in conflict with the attribute of veracity which he must ascribe to the Deity. God could not be the author of such a fraud.

{7} Dr. Bain, Mental Science, pp. 197, 198. In Emotions and Will (3rd Edit.), p. 578, he still denies that "the situation intimates anything as an existence beyond consciousness." This argument in the hands of Dr. Bain, as in those of Berkeley, is based on a deceptive ambiguity in the terms "conceive" and "perceive." We cannot of course perceive an unperceived world, nor can we conceive a world the conception of which is not in the mind; but there is no contradiction or absurdity in the proposition: "A material world of three dimensions has existed for a time unperceived and unthought of by any created being, and then revealed itself to human minds." Dr. Bain's description of the "prevailing doctrine" is only applicable to the theory of mediate perception. It does not refer to Natural Realism, which makes the external material reality the perceived and not the unperceived cause of our cognitions.

{8} It should be carefully borne in mind that in the associationist theory a "possibility of sensation" is not a real actual agent existing out of consciousness. It is as such, non-existent. Its only existence is in the idea or conception by which future experiences are represented. Mill seems frequently to forget this.

{9} Bain. Mental Scieuce, pp. 199, 200.

{10} Mill's Exam, of Hamilton (2nd Edit.), pp. 223, 229, 230.

{11} The defence suggested by some writers, that the scientific psychologist is no more bound to give a metaphysical account of the materials with which he deals than the astronomer, or the geologist, is a mere shallow evasion of the difficulty. Psychology stands here in quite a different position from that of all the physical sciences. Its first duty is to furnish such an exposition of the nature of cognition as will secure an intelligible meaning to the terms employed in all sciences including itself, and assuredly it may not with impunity reduce its own statements to nonsensical absurdities. If it resolves neural currents into modifications of consciousness, it may not then turn round and resolve this consciousness into aspects of the aforesaid currents. If it does so, it is bound at all events to explain the precise significance of the outcome of this interesting dialectical feat. Mill's very just contention against Hamilton is very much to the point here. "When a thinker is compelled by one part of his philosophy to contradict another part, he cannot leave the conflicting assertions standing and throw the responsibility of his scrape on the arduousness of his subject; a palpable self-contradiction is not one of the difficulties which can be adjourned as belonging to a higher department of science." (Exam. pp. 122, 123.)

{12} Critique, translated by Meiklejohn, p. 24.

{13} In maintaining that our developed knowledge of space is a result of experience, a distinction not always realized by Kant should be made between the abstract concept or notion of space in general and the concrete perception of an individual object as extended. The former is an elaborate intellectual product reached by abstraction, reflexion, and generalization, and presupposes many individual -perceptions of concrete extension. The perception, on the other hand, is given, vaguely indeed at first yet truly, in the immediate experience of an extended surface affecting the sense of contact or of sight.

{14} "Kant's fallacy may be put shortly -- What is apodictic (necessary) is a priori; what is a priori is merely subjective (without relation to 'things-in-themselves'); therefore what is apodictic is merely subjective. The first premiss, however, is wrong if a priori is understood in the Kantian sense to mean being independent of all experience. Kant wrongly believes that certitude to be possessed a priori (independently of all experience) which we really attain by a combination of many experiences with one another according to logical laws; and these laws are conditioned by the reference of the subject to the objective reality, and are not a priori forms. He erroneously maintains that all orderly arrangement (both in time and space, and that which is causal) is merely subjective." (Ueberweg's Logic, 28.) Kant has nowhere shown the impossibility of necessary relations being disclosed to the mind in real objective experience.

{15} Physical phenomena find throughout their most complete explanation in the supposition that things-in-themselves exist in a space of three dimensions as we know it. It is at least very doubtful that any other supposition could be so brought into agreement with the facts. We have, therefore, every ground for believing that our conception of substances extended in space of three dimensions does not in some way symbolize things which exist in themselves in quite another way, but truly represents things as they actually exist in three dimensions." (Ueberweg's Logic, § 44, note.) The above line of argument is also urged with great force in Ueberweg's History of Philosophy, Vol. II. pp. 160-166.

{16} Some defenders of Kant assert that he never really intended to make space and time purely subjective, and Mr. Mahaffy replies rather brusquely to Trendelenburg that Kant "never denied their objectivity unless in an absurd sense." (Critical Philosophy, p. 68.) Undoubtedly it is often very difficult to make out Kant's meaning, but if there is a single point on which he seems to be unmistakable it is that space and time are formal, or purely subjective. Whereas sensations of sound and colour are given from without, space and time he holds to be subscribed from within. "Space does not represent any property of objects as things-in-themselves, nor does it represent them in their relations to each other; in other words, space does not represent any determination of objects as attached to the objects themselves, and which would remain even though all subjective conditions of the intuition were abstracted. . . . Space is nothing else than the form of all phenomena of the external sense, that is, the subjective condition of the sensibility under which alone external intuition is possible." (Cf. Critique. Transcend. AEsth. § 4.) Such passages could be multiplied indefinitely. It is a summary. but not very convincing disposal of opponents to simply assert that any other view of space than this is absurd. If it is still maintained that Kant allowed the existence of a noumenal space which suffices for the demands of physical science, then under the shadow of this obscure and elastic term we have admitted an extra-mental extension of three dimensions conditioning the unobserved causes of one sensations, and the chief contention of the Transcendental AEsthetic is abandoned.

{17} Principles of Psychology, § 472.

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