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





Sensation: Sense and Sense-organ. -- The most fundamental and primitive form of conscious life is sensation. Such being the case, sensation cannot, properly speaking, be defined. It may, however, be described as an elementary psychical state aroused in the animated organism by some exciting cause. A sensation is thus a modification, not of the mind alone, nor of the body alone, but of the living being composed of mind and body. The power of experiencing sensations in general is termed sensibility, while the capacity of the living being for a particular species of sensations is called a sense. The special portions of the organism endowed with the property of reacting to appropriate stimuli so as to evoke these particular groups of sensations are called sense-organs. A being capable of sensations is described as sentient, or sensitive; and the term sensuous may be applied to all those mental{1} states which are acts, not of the soul alone, but of the animated organism.

Excitation of Sensation. -- The excitation of a sensation usually comprises three stages. First, there is an action of the physical world external to the organism. This action, transmitted in some form of motion to the sense-organ, gives rise there to the second stage. This consists of a molecular disturbance in the substance of the nerves which is propagated to the brain. Thereupon, a completely new phenomenon, the conscious sensation, is awakened. The nature of the external agencies which arouse sensation is the subject-matter of the science of Physics; the character of the process within the organism which precedes or accompanies the psychical state is studied by the science of Physiology; while the investigation of the conscious operation itself is the function of Psychology. In describing the action of the senses later on, we will say a brief word on the physical and physiological conditions of each in particular, but a few very general remarks on the nature of the physical basis of conscious life as a whole may be suitable here.

The Nervous System. -- The nervous apparatus of the animal organism is two-fold -- the sympathetic system, and the cerebro-spinal system. Whilst the former controls organic or vegetative life, the latter constitutes the bodily machinery of our mental states. The cerebro-spinal system itself is also composed of two parts or subdivisions, the central mass and the branches which ramify throughout the body. The central mass, called the cerebro-spinal axis, is made up of the brain and the spinal cord passing from it down through the backbone. The spinal cord consists of a column of white, fibrous matter, enclosing a core of grey, cellular substance.

From the spinal cord, between every two vertebrae, there issue forth two pairs of nerves. The nerves proceeding from the front of the spinal column are called the anterior, efferent, or motor nerves, inasmuch as they are the channels employed in the transmission of impulses outwards, and are thus the instruments of muscular movement. The nerves coming from the back of the spine are called the afferent, or sensory nerves, because by their means the molecular movements which give rise to sensations, are conveyed inwards from the various organs of the body. The strands of nerves dividing and subdividing as they proceed farther from the trunk branch out into the finest threads through all parts of the skin, so that it is practically impossible to prick any place even with the finest needle without injury to some nerve. The entire surface of the body is thus connected with the brain through the spinal cord by an elaborate telegraph system. (See illustrations at the beginning of the book [Fig. I - III, IV - V, VI, and VII].)

The Brain. -- The brain itself is divided into several portions or organs, the functions of which are, however, in many cases but obscurely apprehended. Amongst the chief are the following:

1. The medulla oblongata, which is situated at the root of the brain where the spinal cord widens out on entering the skull. It is, in fact, the prolongation of the spinal cord. From it proceed the nerves of the face and those governing the actions of the heart and lungs. Hence the fatal nature of injuries in this quarter.

2. Higher up and projecting backwards over this into the lower part of the back of the skull is a large, laminated mass, forming the cerebellum. Its precise functions are still much disputed, but it seems to play an important part in coordinating locomotive action.

3. Above and in front of the medulla oblongata is a quantity of fibrous matter which from its shape and position has been called the "bridge" or pons varolii.

4. Above all there rises the cerebrum or large brain, exceeding in size all the other contents of the skull. It includes several well-differentiated parts lying at its basement, the chief of which are the corpus striatum, the optic thalamus, corpus callosum, and the corpora quadrigemina. The cerebrum consists mainly of a soft, pulpy substance of mixed grey and white matter, the former being composed of vesicles or cells, the latter of fibres. The surface has a very convoluted or crumpled appearance, caused by a large number of fissures. One great furrow, called the median fissure, running from the front to the back of the head, divides the cerebrum into two nearly equal corresponding parts, the right and left hemispheres. Lesser clefts, the chief amongst which are the Sylvian fissure, and the fissure of Rolando, subdivide the two hemispheres into lobes or districts, each containing several convolutions. The nerve-cells in the upper cortical surface of the cerebrum seem to be specially instrumental in the memory, or retention and reproduction of sensory and motor impressions.

The human brain, when it has reached maturity, exceeds that of all the lower animals in the richness of its convolutions. These latter seem to increase the efficiency of the brain as an instrument of the mind, perhaps, by largely augmenting its superficial area. It is thickly interlined throughout with small blood-vessels, and though ordinarily less than one-fortieth of the weight of the body, it receives nearly one-fifth of the whole circulating blood. Mental operations, as is well known, exhaust a great deal of nervous energy, and vigorous intellectual activity requires a plentiful supply of healthy blood to this organ.

Nerves branching into different parts of the head are given off from the centre of the base of the brain in pairs. The first pair, starting from beneath the corpus callosum and proceeding forward form the olfactory nerves. The next pair, having their root a little farther back in the optic thalamus, supply the optic nerves. The remaining nerves have their source in the medulla oblongata. The fifth pair supplies the nerves which control the skin of the face and the muscles of the tongue and jaws. The eighth pair, starting still farther back in the medulla oblongata, constitute the auditory nerve. The ninth pair go to the tongue; and the various nerves issuing from the spinal cord lower down form the tactual and motor nerves of the rest of the body.

Nerve-terminals. -- The external nerve-ends in the several sense-organs are modified and arranged in various ways so as to react in answer to their appropriate excitants. But it is not yet agreed among physiologists how far specialization in the structure of the different parts of the nerve-apparatus is required in order to respond to the different forms of sensori-stimuli.

Sensori-motor action. -- The ordinary process of movement in response to sensations then is of this kind. An impression, e.g., tactual, gustatory, or visual, wrought upon the end-organ of an afferent nerve, is transmitted in some form of motion to a centre in the brain. When it arrives there a sensation is awakened. This state of consciousness now produces an impulse which flows back along a motor nerve and causes some movement. Thus, if a man treads on my foot, I pull it away even involuntarily.

Reflex-action. -- A simpler form of motor-reaction, however, is exhibited in reflex-movement. Here the impression is reflected back along a motor nerve from the spinal cord or some inferior centre before reaching the great terminus in the brain, and there is an appropriate movement in response to the stimuli without the intervening conscious sensation. Thus, tickling the sole of the foot causes convulsive movement even after the spine has been broken and conscious sensibility has been extinguished in the lower part of the body.

Properties of Sensation: Quality, Intensity, Duration. -- The most prominent feature by which sensations of the same or different senses are distinguished from each other, is that of quality. The sensations of sound are thus of a generically different quality from those of smell, while the feeling of blue is of a specifically distinct quality from that of red. These states may also vary in tone, or pleasurableness and painfulness.

Besides differing in quality, sensations may also vary in intensity, and duration. By the intensity of a sensation is understood its vividness, its greater or less strength in consciousness. The degree of intensity depends partly on the force of the objective stimulus, and partly on the vigour of attention. The duration of a sensation means obviously the length of time during which it persists in existence. This is determined mainly by the continuance of the stimulus. The duration of the sensation is not, however, always either equal to or simultaneous with that of the stimulus. A certain brief interval is always required between the irritation of the organ and the birth of the mental state, and the latter continues for a shorter or longer period after the cessation of the former. A certain lapse of time is consequently necessary between two successive excitations in order that there be two distinct sensations. Thus, in the case of sight, if the action of the stimulus be repeated oftener than five times in the second, it ceases to be apprehended as a series of separate events, and instead, one continuous sensation is aroused. The ear can distinguish as many as fifteen successive vibrations in the second, while the recuperative power of taste and smell, after each excitation, is far lower than that of sight.

Composite stimuli. -- It is erroneous, however, to speak of the continuous sensation produced by these repeated excitations as a compound sensation arising from the combination of a number of simple sensations. It is only by an inaccurate metaphor that unextended mental states can be described as blending, or mixing, after the manner of liquids or gasses; and there is, moreover, nothing to show that the supposed constituent elementary states ever came into existence. The simplest and briefest sensation has for its physical condition a neural process, divisible into parts; it would, however, be absurd to speak of it as composite, on this account. In the case of a continuous sensation of sound, or colour, arising from an intermittent stimulus, the physical and physiological conditions may be more complicated, but the mental state felt to be simple must be described by the psychologist as such.{2}

Somewhat similarly, in the case of touch, a certain interval of space, variable in different portions of the body, must exist between the parts of the organism affected by two stimuli, in order that these may be felt as distinct. The capacity of sensation for variation in intensity and duration has suggested in recent times the attempt to secure exact quantitative measurement of mental phenomena, and the title of Psychophysics has been allotted to this line of investigation.

Cognitive character of Sensation. -- The features hitherto described, including even pleasantness or painfulness, are merely aspects or accidental properties of sensation. Its essential nature lies in its cognitive quality. The intensity, duration, and emotional tone of a sensation, exist only as they are known. They are of a variable and adjectival nature. They determine and modify, but they do not constitute the essence of a sensation. A sensation is in itself an elementary mode of consciousness of a cognitional character. Knowledge, however, may have reference either to extra-organic, or to intra-organic objects and events. We may be cognizant of something other than ourselves, or of the states of our own sentient organism, and different senses stand higher and lower in regard to these different fields. In sight, in the muscular sense,{3} and in the tactual sensations of pressure, knowledge of external reality is the prominent feature; in hearing, taste, smell, and the organic feelings, the sensation is a cognition, which originally bore a subjective character. In the case of these latter faculties, the pleasurable or painful aspects of sensations frequently rise to great importance; and on some occasions the sensation becomes mainly a cognition of pain, or, more rarely, of pleasure.

Sensation and Perception. -- This distinction between the objective and subjective import of the sentient act has caused the two terms, sensation and perception, to be contrasted with each other. Sensation, as thus opposed to perception, is variously defined to be, the modification of the sense viewed merely as a subjective state, the consciousness of an affection of the organism, or the feeling of pleasure or pain awakened by the stimulus. Perception{4} is described as the objective knowledge, the apprehension of external reality given in the sentient act; or, as the act by which we localize or project a sensation or cluster of sensations, actual and possible, into the external world.

This separation of the two terms is convenient for bringing out the difference between the developed form of cognition exhibited by sense in mature life, and the vague kind of apprehension afforded in the earlier acts of the sentient powers: but the distinction is one of degree, not of kind. In the most rudimentary sensations of pressure and of colour, there is a cognition of something other than self, and though rude and indefinite in character, this is still an act of objective knowledge. Consequently, there is already here perception, in the modern signification of the term. This vague act receives exacter definition as we advance, and in later years the quality perceived by the sense is cognized as situated in a determinate place, and accompanied by other qualities. Such further determinations, are, however, the result of other sensations, and if no one of them revealed external reality to us, the aggregate could not do so. This subject will be better understood when we come to treat of the nature of Perception. Some writers define Sensation as the feeling of pleasure or pain attached to an act of sensuous apprehension, but very few, if any, adhere consistently to this interpretation. When, for instance, the sensations of the different senses are spoken of, and their various properties, quality, intensity, tone, duration, and the rest, are described by psychologists, sensation does not mean the pleasurable or painful aspect of certain mental states, but these states themselves. It is only when used in this narrow signification, as a feeling of pleasure or pain, that sensation and perception can be held within certain limits to stand in an inverse relation to each other.{5}

The modification of a sensuous faculty is thus, in its simplest form, of a percipient character, and in the case of vision and touch, the sensation from the beginning possesses a certain objective reference. A sensation viewed in this way as a modification by which the mind is made cognizant of a material quality of an object, was called by the schoolmen a species sensibilis.

The Scholastic Doctrine of Species. -- The doctrine of species has been attacked and ridiculed by many modern writers, and this in a manner which shows how widespread and profound, even amongst students of philosophy, is the ignorance regarding the most familiar terms of scholastic writers. Democritus and Epicurus formerly taught that we know objects by means of minute representative images which stream off from their surface, and pass into our soul through the channels of the senses. The Latin word species, meaning an image, was used by their Roman disciples to signify these volatile images. Aristotle and his followers, however, rejected the theory of a physical efflux of species, and taught instead, that objects effected modifications in the mind by acting on the sense-organs through motions in the intervening media. The term species was later on employed to denote these modifications by which the mind is made to apprehend the exterior object. In this sense, which is that accepted by the greatest philosophers of the middle ages, such as St. Thomas, Albertus Magnus, and Scotus, the species is not an entity which has immigrated into the mind from the object, but a modification or disposition awakened in the mind by the action of the object. They teach, moreover, that this mental modification is not what is primarily perceived in the act of simple apprehension. The mind, they hold, directly tends towards the objective reality; and only by a reflex or concomitant act does it cognize the mental state as such. With them, Species non est id quod primo percipitur, sed id quo res percipitur. It is the medium vel principium quo, non ex quo, res cognoscitur. In other words, the species is not an intermediate representation from which the mind infers the object, but a psychical modification by which the mind is likened, or conformed, to the object, and thus determined to cognize it.{6}

Intentionalis. -- The adjective intentionalis was added to the term species to signify that the cognition, though truly reflecting the external object, does not resemble it in nature. The mental modification was held to be merely a psychical or spiritual expression of the material thing. Resemblance is of many kinds. A photograph, or a statue, is, in a certain sense, utterly unlike a man formed of flesh and blood; the blind man's representation of a circle by the sense of touch, is very different from the visual image of the same figure; the intellectual ideas aroused by the words, "equality," "colour,' "square," must be widely divergent from both the image and the reality to which they correspond. Yet, in spite of these unlikenesses, there exist genuine relations of similarity between such pairs of things as those just mentioned. The scholastic writers adopting this view, taught that our knowledge, although in itself, as a mental activity, opposed in nature to material reality, does, nevertheless, truly mirror the surrounding world. They held that though neither the tactual nor the visual image resembles in nature the brass circular substance presented to the sense, yet both accurately reflect and are truly like the external reality; and they called these mental expressions of the object species intentionales.

Species sensibiles et intelligibiles. -- Furthermore as the shoolmen held the human mind to he capable of two essentially distinct kinds of cognition, sensuous and intellectual, they termed the apprehensive acts of the former species sensibiles, of the latter species intelligibiles vel intellectuales. In the genesis of the species they distinguished two moments or stages. The modification of the sensuous faculty, viewed as an impression wrought in the mind by the action of the object, was named the species impressa. The reaction of the mind as an act of cognitive consciousness was styled the species expressa. The latter term designated the sensation considered as a completed and perfect act of consciousness elicited by the soul; the former indicated the earlier stage of the process, the alteration in the condition of the mind looked at as an effect of the action of the object.{7} The species proper, however, whether impressa or expressa, was an affection of the mind. The term species corporalis was sometimes used to signify the physical impression or movement produced by the object in the organism, but the strict meaning of the word species, and the only meaning of the term species intentionalis, was the mental state. Thus, neither the image of the object depicted on the retina of the eye, nor the nervous disturbance propagated thence to the brain, but the conscious act finally awakened, was held to be the true species or species intentionalis.

True doctrine. -- Rejecting the interpretation of the species as roving images, and every theory conceiving them as representations mediating between the object and the cognitive faculty, the thought embodied in the doctrine is thoroughly sound. Unless we are prepared to maintain that our soul is born with all its future knowledge ready made, and wrapped up in innate ideas, we must allow that the physical world does somehow or other act on our faculties, and that our perceptions are due to the influence of material objects upon us. The mind does not determine all its own modifications, and the strongest volition is unable to make the deaf man hear a word, or the blind man see a colour. But this is to admit that the faculty is stirred into conscious life and informed by dispositions wrought in it by the perceived ohject. Further, unless we are ready to adopt the position of absolute scepticism, we must hold that knowledge does somehow correspond to reality. There is not a merely arbitrary connexion between the object and its apprehension. The latter is a true, though psychical expression of the former. This subject will be more fully dealt with hereafter, but we have said enough to justify the doctrine of species intentionales, as understood by St. Thomas, and the leading philosophers of the school.{8} The modern writer may prefer to describe the perception of a triangle as a modification of the mind mirroring or reflecting in terms of consciousness the external object, but this is only the old doctrine in other phraseology.

Experimental Psychology. Psycho-physics. --

The measurement of mental states. -- If one ounce be added to a weight of three ounces placed on our hand resting upon the table, we can just distinguish the new sensation from the old. A single voice also makes a perceptible increase in the sound when added to a musical trio. If, however, we add a single unit to a weight of thirty ounces or to a chorus of twenty voices, no difference can be felt. By observing and comparing sensations produced by stimuli varying in intensity, a German physiologist, Weber (1834), showed that the increment necessary to be added to a given stimulus in order to awaken a sensation consciously distinguishable from the former sensation varies with the force of the former stimulus. Thus, if d represent the minimum increment that must be added to a stimulus of the force Z in order to be felt, there will be needed an increment of 2 d to a stimulus of 2 Z to be perceived, and in general n d must be added to n Z to cause the minimum appreciable difference in the resulting sensation. In other words, the minimum appreciable increment in a physical stimulus bears a constant ratio to that stimulus, though this ratio differs for several senses. This ratio, d/Z, is found by some observers (though others give different results) to be in sensations of light 1/100, in muscular sensations 1/17 in sensations of pressure, of warmth, and sound 1/3. The generalization has been called Weber's Law.

Continuing Weber's investigations, Fechner (1861) formulated the law in a more complete shape. His main object was to find some fixed unit by which to measure sensations. He believed he had discovered such a unit in the least observable difference between two sensations. This he supposes to be a constant quantity for the same sense, whatever be the intensity of the sensations, excluding extreme limits. Any sensation of intensity. N may be conceived, he held, as equivalent to N of these units, and may according be mathematically calculated in terms of the stimulus. To take an example: "If stimulus A just falls short ot producing a sensation, and if r be the percentage of itself which must be added to it to get a sensation which is barely perceptible -- call this sensation 1 -- then we should have the series of sensation-numbers corresponding to their several stimuli, as follows:

Sensation 0 = stimulus A
Sensation 1 = stimulus A (1 + r)
Sensation 2 = stimulus A (1 + r)2
Sensation 3 = stimulus A (1 + r)3
Sensation n = stimulus A (1 + r)n

The sensations here form an arithmetical series, and the stimuli a geometrical series. . . . So that we may truly say (assuming our facts to be so far correct) that the sensations vary in the same proportion as the logarithms of their respective stimuli. And we can thereupon proceed to compute the number of units in any given sensation (considering the unit of sensation to be equal to the just perceptible increment above zero, and the unit of stimulus to be equal to the increment of stimulus r, which brings this about) by multiplying the logarithm of the stimulus by a constant factor which must vary with the particular kind of sensation in question. If we call the stimulus R and the constant factor C, we get the formula:

S = C Log. R,

which is what Fechner calls the Psychophysische Maasformel."{9} The outcome, accordingly, of these investigations is summed up in the so-called psycho-physical law: To increase the intensity of a sensation in arithmetical progression, e.g., as 1, 2, 3, 4, the stimulus must be increased in geometrical progression, e.g., as 1, 2, 4, 8, or, the sensation increases as the logarithm of the stimulus.

The absolute sensibility of an organ, or part of an organ, is measured by the minimum perceptible stimulus, or that which just rises above the threshold of consciousness.{10} The absolute sensibility of the skin to tactual pressure varies in different parts from .002 to .015 of a gramme; the absolute sensibility of the skin to changes of temperature varies from .2 to .9 degrees Centigrade, the skin being about 30 degrees Cent.; that of hearing is the sound of a ball of cork, 1 milligramme weight, falling on a vibrating plate from a height of 1 millimetre, at a distance of 91 mm. from the ear; that of sight, the 1/300 of the light reflected by white paper under the full moon.{11}

When the stimulus has reached a certain intensity, further increase produces no appreciable difference in the sensation. This maximum stimulus measures the height of sensibility of the sense; and the interval between the threshold and this point has been termed the range of the sensibility of the sense.

Criticism. -- The professed object of this line of investigation is to introduce quantitative measurement into Phenomenal Psychology, and so to reduce this branch of mental philosophy to the condition of an exact science. Now, whilst we readily admit that real care and ingenuity has been exhibited in carrying out these experiments, and that many of the facts established are curious and interesting, we believe that the advocates of Psycho-physics mistake and seriously exaggerate the value of that branch of study. 1. In the first place, it may be objected that the fundamental assumption on which Fechner's scheme of measurement rests is untenable. The so-called least observable differences of sensations, or more correctly the judicial acts by which we discriminate barely distinguishable impressions, are not constant equal quantities of consciousness. Still less can it be proved that every sensation is a definite multiplication of such units.{12} 2. Next, it is only a small part, and that the lowest and most unimportant part of mental life, that can be at all approached by the instruments of this science. Emotions, volitions, and all intellectual processes are obviously beyond the reach of any form of quantitative measurement. Even, then, if psycho-physics had attained the utmost hopes of its supporters, and if -- what appears equally unlikely -- these supporters became agreed as to their results, our knowledge of mental life would not really be thereby much advanced. 3. Again, there may be raised an objection against the conclusions of psychophysicists even within the restricted sphere of sensational consciousness, an objection which strikes at the possibility of any kind of quantitative estimate of mental phenomena. An assumption, involved in all Fechner's experiments, and lying at the root of his chief psychological law, implies that while sensation increases in quantity or intensity, the quality remains unaffected. A locomotive of twenty-horse power can drag a load twice as heavy as an engine of ten-horse power. The force exerted in such a case may be rightly described as double in quantity yet similar in quality. But we can hardly say this as regards the energies of mental life. Sensations of light, sound, temperature, and the rest, increased in intensity, do not appear to preserve the same quality of consciousness. The transition from black to white, from hot to cold, from the trickling of the fountain to the roar of the waterfall, is not merely a variation in quantity. In small increments, the alteration in quality may escape notice, but when the effects of large changes in the degree of the stimulus are compared, introspection seems to affirm changes of quality as well as of quantity.

4. Finally, these difficulties are reinforced by serious attacks from careful observers, who question the truth of the alleged results on the evidence of direct experience. Thus, Hering, for example, rejects the Weber-Fechner generalization on the grounds, (a) that admittedly its application has to he limited to a very narrow range above and below normal stimulation, and (b) that it is completely "inapplicable either to taste or smell, to heat, to weight, or to sound, and that therefore it has not the character of a general law of sensibility."{13}

Interpretation of the Weber-Fechner Law. -- Why, it may be asked, does the sensation increase more slowly than its objective excitant? Fechner answers that his generalization in an ultimate law describing the relation between physical stimuli and psychical reaction, or between body and soul. The intensity of the nervous change transmitted to the brain increases, he supposes, in direct proportion to the physical stimulus, but the sensation only in proportion to the logarithm of the latter. It must, therefore, be conceived as an ultimate psycho-physical law for which no further explanation can be demanded.

Others give a physiological interpretation to the generalization of the facts in so far as this holds good. It is, they assert, not an ultimate expression of the relation between mental and material action, but a law describing the relations between the external physical stimulus and the nervous action which reaches the brain. The conscious reaction in this view increases in direct proportion to the intensity of the final physiological stimulus, but the latter increases more slowly than the physical stimulus, owing to the augmentation of resistance and friction as the sphere of nervous disturbance becomes larger.

Finally, others seek to explain the law psychologically, maintaining that it expresses neither the relation between the physical and the psychical change, nor between the former and physiological action, but between the sensations and our powers of discriminating them. All appreciation, according to these writers, is relative to existing states. The differences between mental states have their value determined by their relation to these states, diminishing in proportion to the intensity of the latter.{14}

Whilst the reality of the law is subject to such serious dispute, speculation as to its interpretation appears to us neither very hopeful nor profitable, but the physiological explanation seems to give a sufficient account of the facts.

Psychometry: Reaction-time. -- If a harpoon be stuck in the tail of a whale an appreciable interval elapses before the tail is moved. The impression, in fact requires time to be transmitted along an afferent nerve to the whale's brain before the whale becomes conscious of the pain, and another period is needed for the transmission of an impulse back from the brain along a motor nerve to set the tail in motion. The whole interval is called reaction-time. A similar phenomenon is observed in the case of human beings in regard to impressions on the several senses. In recent years ingenious psycho-metrical instruments have been invented, and a great number of elaborate experiments have been made to determine accurately the reaction-time with respect to different kinds of sensation. The general plan pervading the various methods of experiment is the stimulation of some sense-organ to which the subject responds by a sign the instant he apprehends the sensation.{15} The experiment can be varied so as to involve simpler or more complex operations. Thus the subject, who is blindfolded, is asked to press an electric button as soon as he feels a tap on either knee, whilst a finely graduated timekeeper measures, to the one-hundredth part of a second, the interval between the tap and the signal. Next he is asked to press the button only when the right knee is tapped, remaining quiet if the left is touched; or he is requested to signal with the other hand if he feels the sensation in his left leg. The act of choice here introduced considerably lengthens the process. Similar experiments are made in regard to the time occupied in apprehending and discriminating various sensations of colour, sound, taste, and smell.

The entire process between the impression and the motor sign has been analyzed into several stages, amongst which the following may be easily distinguished:{16} (1) The excitation of the end-organ of the sense sufficiently to start the neural change. (2) The conduction of this neural change along the centripetal or afferent nerves to the brain. (3) The transformation of the sensory impression into the motor impulse. (4) The transmission of this motor change back along efferent nerves to the appropriate muscle. (5) The excitation and contraction of this muscle in the signalling action.

Of these stages only the third is a psycho-physical event. All the others are physiological, and as their duration can be approximately determined by various experiments and then eliminated, the length of the strictly psycho-physical portion of the whole reaction it is alleged, may be estimated.

Wundt gives as average total reaction-time of a series of experiments, for impressions of sound, 0.128 of a second; for light, 0.175; for touch sensations, 0.188. But Exner, Hirsch, and others give different figures. Study of these investigations goes to prove that the reaction-time varies much with different individuals. On this fact is based the "personal equations" of different observers which have to be taken into account in certain delicate astronomical observations. Further, it seems clear that practice shortens the reaction-time very considerably, and that expectant attention also diminishes it. Fatigue increases it, whilst the weather, the health of the individual and the nature of the stimulus also modify the rapidity of reaction.

The hope of attaining exact quantitative measurement of mental activities has, however, been growing fainter among even the most ardent experimentalists in recent years, and the truth of the old view that introspection must be the primary source of information even in experimental psychology, is reasserting itself every day. The measurements now made are as a rule subordinate and mainly intended by the use of averages to secure rough appreciations.

Under this new movement the field of research has been widened, and much industry and ingenuity have been exerted to bring the higher mental processes within the range of these more elastic experimental methods. From mere sensation experiments have passed on to the study of memory, the rapidity of acquisition, the duration of retention, the celerity of acts of recognition and judgment, the working of association, the vividness and precision of imagination and the relation of images and words to thought. The insecurity of the results attained up to this is, however, evinced by the keenness of the controversies respecting most of the facts claimed to be established. Efforts have also been made to apply the experimental methods in the region of fhe will, and interesting investigations have been carried out, especially at Louvain, on volitional activity. Still it has to he admitted that hitherto, at all events, the total outcome of the Herculean labours expended on these researches is, from the psychological standpoint, distressingly small, and particularly disappointing is the complete failure to shed any really new light on the old philosophical problems.

More interesting and fruitful have been the researches in pathological psychology. Abnormal mental activities, though very liable to misinterpretation and false generalizations are often illuminative, and the systematized observations now carried on in this field may result in a real gain to the science of the mind.

Possibly the main benefits which will accrue to psychology from these new departments of research may be the raising of the standard of precision and the stimulus and encouragement to seek increased accuracy in the use of ordinary introspective observation and analysis of mental operations. Moreover, these studies have already led to an improved knowledge of the nervous system and the material mechanism immediately subservient to our mental life.

Readings. -- On the brain and nervous system, Ladd's Elements of Physiological Psychology. On Sensation and Perception, Hamilton, Metaphysics, vol. II. pp. 91-97. The subject of Species is treated in all the Latin scholastic manuals of Psychology. On General Experimental Psychology, see Titchener, Experimental Psychology (N. York, 1901-06). On Memory: Ebbinghaus, Über das Gedachtniss (Leipzic, 1885): Neumann, Vorlesungen, Experimentelle Pädagogik (Leipzic, 1907); On Thought: Bühler, Tatsachen and Probleme zu einer Psychologie der Denkvorgänge (Archiv. f. die ges. Psychol. 1907 and 1908) Aveling, Consciousness of the Universal (London, 1912); On the Will: Michotte and Prum, Etude Experimentale sur le choix volontaire (Louvain, 1911) ; Boyd-Barret, Motive Force and Motivation-Tracks (London and Louvain, 1911); also the Annales de l'Institute Superieur de Philosophie (Louvain since 1912).

{1} We employ the word mental, as equivalent to conscions. In this sense, it is applicable to all states of consciousness, whether cognitive or appetitive, sensuous or supra-sensuous. The usage of those scholastic writers who would make this adjective synonymous with intellectual, seems to us inconveniently narrow, and too much opposed to common language.

{2} The 'objective' analysis of mental states by Mr. Spencer and M. Tame is thus illusory. If states which consciousness -- the only possible witness concerning such facts -- declares to he simple, are to be reduced to units of the character of nervous shocks, because the action of the physical agent is of a composite character, then we certainly cannot stop at the feeling of a "shock," as the unit. The briefest and simplest sensation of the colour of violet, which involves between six and seven hundred billions of vibrations in the second, must be resolved into an incredible number of unconscious units of consciousness, for the existence of none of which, of course, is there any evidence. A knowledge of the physical conditions of mental states is valuable, but conscious elements affirmed to be simple by introspection, must be accepted as such by the psychologist. "Mental facts," as Mr. Mark Baldwin urges, "are simple states, and they are nothing independentiy of the mind whose states they are." (Cf. Senses and Intellect, pp. 98-106; also Dr. Mivart, Nature and Thought, 2nd Edit., pp. 89-92.) See also pp. 510-512, below.

{3} This term is used to denote the power of experiencing sensations of resistance or impeded energy and movement. Its nature will be discussed in the next chapter.

{4} The word perception, or rather, the Latin verb percipere, was originally used in a wide sense to denote any form of apprehension or comprehension, whether sensuous or intellectual. Later on, it became limited to sensuous apprehension, and was employed by Reid, in contrast to the term sensation, to designate the sensuous cognition of something as external to us. Sensation originally meant the process of sensuous apprehension considered as revealing to us both itself as a subjective state, and the objective quality to which it corresponded. By Reid it was confined to the former signification, and thus explained: "The agreeable odour (of a rose) which, I feel, considered by itself without relation to any external object, is merely a sensation. Perception has always an external object, and the object in this case is that quality in the rose which I discern by the sense of smell." The later sensationalists (e.g. Mr. Sully, Outlines, c. vi.), inverting the doctrine of Reid and Hamilton, that perception is the apprehension of a real external quality, describe this act as an ejection or projection out of the mind of a sensation carrying with it a cluster of faint representations of other past sensations, the whole being "solidified" or "integrated' in the form of an object. On the terms sensation and perception cf. Hamilton, On Reid, Note D, also Metaph. Vol. II. 93-97.

{5} Hamilton explains Reid to mean by perception, "the objective knowledge we have of an external reality through the senses; by sensation, the subjective feeling of pleasure or pain with which the organic operation of sense is accompanied," and adopting this view he enunciated the law that above a certain point the stronger the sensation the weaker the perception, and vice versa. He seeks to establish this general opposition by a comparison (a) of the several senses, and (b) of different impressions within the same sense. Confined to sensuous apprehension, the formula seems to be approximately true, although it is pain rather than pleasure which interferes with cognition. As a generalization applicable to higher intellectual forms of cognitive activity, it does not hold. Consciousness is not, as Hamilton seems to imply, a fixed quantity where increase in cognition involves decrease in feeling. This is in direct opposition to the doctrine adopted by Hamilton himself from Aristotle, that pleasure is a reflex of mental energy. In the view of the Greek philosopher, keen and intense pleasure accompanies Vigorous intellectual activity and the greatest and best pleasure is the necessary sequela of the exercise of the highest form of cognitive energy. (Cf. Hamilton, Metaph. pp. 93-105.)

{6} If the primary object of cognition were the mind's own unextended modification, idealism and relativism would be inevitable. "Quidam posuerunt, quod vires, quae sunt in nobis cognoscitivae nihil cognoscunt, nisi proprias passiones, puta, quod senses non sentit nisi passionem sui organi, et secundum hoc intellectus nihil intelligit, nisi suam passionem, scilicet speciem intelligibilem in se receptam; et secundum hoc species hujusmodi est ipsum quod intelligitur. Sed haec opinio manifeste apparet falsa ex duobus. Primo quidem, quia eadem sunt quae intelligimus, et de quibus sunt scientiae si igitur ea, quae intelligimus essent solum species quae sunt in anima, sequeretur quod scientiae omnes non essent de rebus, quae sunt extra animam ned solum de speciebus intelligibilibus quae sunt in anima. Secundo, quia sequeretur error antiquorum dicentium, omne quod videtur, esse verum; et similiter quod contradictoriae essent simul verae; si enim potentia non cognoscit nisi propriam passionem, de ea solum judicat . . . puta si gustus non sentit nisi propriam passionem, cum aliquis habens sanum gustum judicat mel esse dulce, vere judicabit; et similiter si ille, qui habet gustum infectum, judicet mel esse amarum vere judicabit; uterque enim judicabit secundum quod gustus ejus afficitur. . . . Et ideo dicendum est quod species intelligibilis se habet ad intellectum ut quo intelligit intellectus. . . .

"Sed quia intellectus supra seipsum reflectitur, secundum eandem reflexionem' intelligit et suum intelligere et speciem qua intelligit. Et sic species intellecta secundario eat id quod intelligitur. Sed id quod intelligitur primo est res cujus species intelligibilis est similitude." (Sum. Ia. q. 85. a. 2.)

{7} Any real distinction between the two species may be disputed, but that the alteration or modification wrought in the soul by the act of perception must persist in some form, is established by the facility of representation and recognition.

{8} Even Hamilton confounds the maintenance of species with the doctrine of mediate perception, and so looks on St. Thomas and the great body of the schoolmen as representationalists or hypothetical realists. (Cf. On Reid, Note M, pp. 852-857.) The passage cited from St. Thomas, p. 52, refutes the charge. For a full treatment of the subject, see Sanseverino, Dynamilogia, pp. 390-400; Pesch, Instit. Psych. 472; Boedder, Psych. Rationalis, 255-260; Schiffini, Psychologia, 302.

{9} James, Vol. I. pp. 538, 539.

{10} This "absolute sensibility" is ascertained by gradually increasing an imperceptible stimulus, or by diminishing a clearly perceptible one till it just reaches the margin. Similarly, the minimum observable difference can be obtained either by starting with two equal stimuli and progressively unequalizing them, or by beginning with easily distinguishable stimuli and reducing the difference until they are barely discernible. To eliminate the errors incidental to such delicate appreciations, the experiments are multiplied and combined in various ways, and also corrected by the ordinary scientific methods of measurement such as that of average errors or that of correct and mistaken cases. Thus, if two stimuli differ by less than the minimum observable difference, true and false guesses as to which is the stronger tend to be about equal; but as soon as the difference begins to exceed the minimum limit, the correct judgments rapidly exceed those which are erroneous. (See E. Scripture, The New Psychology, c. iii.; Ladd. op. cit. p. 364; James, ibid. Vol. I. pp. 540, 541.)

{11} Unfortunately the figures given by different observers vary.

{12} Cf. Ladd, ibid. pp. 361, 362; Lotze, Metaphysic, §§ 258, 259; ibid. p. 546. On the other side, cf. Wundt, Human and Animal Psychology (Eng. Trans.), pp. 20-60.

{13} Ribot, La Psychologie Allemande, p. 196. Chapter v. of that work contains a good resumé of the subject. See also, Ladd's Physiological Psychology, Pt. II. c. v. The reader of that chapter will notice how much disagreement prevails regarding the figures. Of scholastic writers on this subject, see Gutberlet, Die Psychologie, pp. 34-45; Mercier, Psychologie, pp. 148-154; Farges, Le Cerveau et l'Ame, pp. 209-226.

{14} Cf. Wundt, ibid. pp. 59-64.

{15} A full description and numerous illustratioas of these variouS psycho-metrical machines are given by E. Scripture, op. cit.

{16} Cf. Ladd, op. cit. p. 470; James, op. cit. p. 88.

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