University of Notre Dame
Jacques Maritain Center   

Philosophy and the Unity of the Sciences

Address by Jacques Maritain at the twenty-seventh annual meeting of the American Catholic Philosophical Association, University of Notre Dame, 7 April 1953. Transcribed from a typewritten manuscript.


1. It is an obvious fact that the sciences, in proportion as they have achieved the admirable developments which characterize them in modern times, have tended toward greater and greater multiplicity. Great scientific disciplines have divided and are dividing into parts each of which is enough to absorb the whole competence of a given scientist. And this process of specialization will normally continue. Of themselves, the sciences tend to divide and multiply, so to speak ad infinitum.

Will the human mind itself enter, as a result, a state of intellectual atomization? If the sciences are to achieve some kind of unity, this cannot be expected at the proper level of the sciences themselves. It can be obtained only at the level of some superordinate knowledge, which uses other ways and approaches to conquer intellectually reality. Such knowledge, superordinate to the sciences, is philosophy.

Summing up what I have tried to elucidate in other essays, I would say that "it is necessary to recognize two essentially distinct ways of analyzing the world of sense-preceivable reality and of building the concepts required for this. The first way is by a non-ontological analysis, an "empiriological" analysis of the real. This is the scope of scientific knowledge. The second way is by an ontological analysis of the real. This is the scope of philosophical knowledge... Science resolves its concepts and its definitions in the observable and the measurable as such. Philosophy resolves its concepts and its definitions in the intelligible being." Thus, "in order to master becoming and the flux of phenomena, science works, so to speak, against the grain of the natural tendencies of the intellect, and uses, as its own instruments, explanatory symbols which are ideal entities (entia rationis) founded on reality, above all, mathematical entities built on the observations and mesurements collected by the senses." Science is a kind of knowledge which is constructive in type, it constructs on the data of experience systems of symbols the emergent results of which are to be verified by facts. "On this condition, the human mind can scientifically dominate becoming and sense- perceivalbe phenomena, but, at the same time, it gives up any hope of grasping the inner being of things." [The Range of Reason p. 8].

On the other hand, philosophy is a kind of knowledge which is perceptive in type; its instruments - concepts provided by eidetic abstraction which are determinations of the primary concept, the concept of being, and judgments which are related to the primary intuition of the first principlesa - are instruments through which the intellect has a vision or perception of its own. For the intellect sees. More or less confusedly or distinctly, an in a parcelled-out manner, the intellect sees what Aristotle called the to ti en einai, the intelligible features of that which the things touched by our senses are. And by virtue of the inner necessities involved in the concepts themselves, - either, in the primus modus dicendi per se, because the predicate pertains to the notion of the Subject, or, in the secundus modus dicendi per se, because the subject pertains to the notion of the Predicate, which is its propria passio or immediate property, - the intellect sees immediately or intuitively the first principles of reason. And the reasoning is a transference of evidence which makes the intellect see the necessity of the conclusion by virtue of what is seen in the premises. All this is grounded in the primary intuition, which is the intellectual intuition of being, or of the act of existing exercized by things.

Such are the instruments proper to philosophy, when the intellect's power of vision, oriented as it is toward the intelligible being, is organized in an articulate system of concepts, definitions and assertions, each of which is like an immaterial lens through which a given aspect of being is grasped.

2. I just emphasized the distinction between science and philosophy. This distinction was overlooked as a rule by the ancients. At the dawn of philosophical thought in Greece, - when the great Ionian poet-physicists explained everything by Water or Air or Fire or Polemos or the plenum and the void, - the ideal of a knowledge both one and universal was pursued by the human mind, still dreaming rather than conceptualizing wisdom, at the level of the interpretation of becoming and the world of sense appearances. Then, after Anaxagoras, Parmenides and Socrates, the ideal of the one and the universal knowledge was transferred, with Plato, to the level of philosophy. But for Plato philosophy or genuine rational knowledge dealt only with the supra-temporal necessity of Numbers and self- subsisting Ideas, the world of becoming could not be the object of any science or philosophy; only a kind of symbolic knowledge, through myths and poetic imagery, was able to tell us something of the transient reflections of being on non-being. Aristotle realized that scientific knowledge of movement and of becoming was possible, and thus he founded Physics, or the Philosophy of Nature, as distinct from Metaphysics. This was a step of fundamental import. But for Aristotle the philosophy of nature absorbed in itself all our knowledge of nature; on the one hand it had to analyze the essence of material things and recongize the existence of the first Mover, and on the other it had to explain the datail of phenomena.

So the formal perspective was everywhere the perspective of being. Outside of certain special fields like astronomy, optics, harmony, the study of phenomena consisted for the ancients of general interpretations conducted under the light of philosophy, and essentially oriented toward an ontolotical knwledge and analysis of things; interpretations which were less certain, more dependent on simple probability and closer to simple opinion, the nearer they came to the detail of phenomena. And this continued to be the case in the Middle Ages and right down to the seventeenth century. In the ancient treatises on natural philosophy we find long discussions about meteors, explanations of rainbows, snow crystals, etc. The questions which are called scientific today did not constitute a specifically distinct discipline, they only dealt with particular applications requested by empirical facts, and were just a chapter of philosophy. Descartes held the same position. With him the royal power, as to the explanation of nature, shifted from metaphysics to mathematics, and thus Descartes was, in actual fact, at the origin of the modern distinction between science and philosophy; but he himself did not make the distinction. His geometrical physics, his optics, his theory of tourbillons were for him the true philosophy of nature, and his treatise on meteors was a book of philosophy. He even emphasized the unity of our intellectual approach to reality still more strongly than Aristotle and the mediaeval doctors did.

It is my conviction that the Thomists of our day have not only to accept willy nilly the modern distinction between science and philosophy, but to recognize its specific character and to establish it on the rational foundation of a sound critique of knowledge. Nothing could be more useless and detrimental for Scholastic philosophy than futile attempts at any kind of concordiam which would make Einstein and Aristotle bed-fellows occupied with the same kind of truth and explanation. Thomist philosophy and modern science do not need to be reconciled, they are in natural accordance, on the condition that we do not dream of making the physicist subservient to our philosophy, or our philosophy subservient to the physicist, and that we do not try to build a philosophy of nature on Einsteinian space-time or to derive free will from physical indeterminism, or, conversely, that we do not feel obliged by nuclear physics to reject the philosophical theory of matter and form. We have, on the contrary, to distinguish the empiriological approach and the ontological approach more sharply than modern philosophers usually do, and, thanks to a critical elucidatino of the degrees of knowledge, to stress the fact that the scientist and the philosopher can both progress ad infinitum in their own realm without coming into conflict because they do not fish in the same waters. This is the only way to do full justice both to science and philosophy, and, at the same time, genuinely to free Thomist philosophy from its accidental association with an imagery provided by a pseudo-philosophical explanation of phenomena which has been ruined.

Modern science has done philosophy an immense service in delivering it from the essentially alien concern, which had oppressed it for so long, of explaining phenomena. If we consider the requirements of objective intelligibility, there is no necessary link between the mechanics, physics and astronomy of the ancients and the metaphysics and natural philosophy of the scholastic tradition. The whole edifice of the edxperimental science of the ancients could fall in ruins; this immense wreck could seem to hasty minds as if it were the ruin of all the ancients had thought: in reality their metaphysics and their philosophy of nature, in their essential principles, have been no more affected than the spiritual soul is altered by the dissolution of the body. I am ready to admit that the position of a soul without a body hre on earth is rather uncomfortable, and that it is urgent for Thomist philosophy, once the above-mentioned purification has taken place, to recover its organic relations with the grand totality, the life and activity of the sciences, - this I shall try to point out in the second part of my paper. But I submit, on the other hand, that we do not have to conceive these organic relations with contemporary science in an uncritical and illusory way, beginning again the old story which was so harmful to our forefathers. This would be all the more unforgivable since contemporary science itself is fully aware (as opposed to the science of the ancients) of its own phenomenal, non- ontological and non-philosophical character.

I alluded a moment ago to the way in which Descartes conceived the unity of the sciences. Knowledge, for him, possessed the very same unity as the mind, that is, the human soul or the human self. In other words he did not admit that specific diversity in nature which Thomas Aquinas recognized in the various intellectual virtues: because for Descartes there were no such tings as accidents, qualities or virtues distinct from the substance of the soul. As a result, the diversity of habitus or virtues became only a diversity of functions in a single whole, essentially one and indivisible; the function of metaphysics was to guarantee the validity of knowledge; and the very body of the knowledge itself was that scientia mirabilis which had been revealed to him in his famous dream of November 1619, the universal mathematics, the mathematical explanation of the entire physical world.

This theory of Descartes proved powerless, and even senseless, with respect to the real development of the sciences, which, as I observed at the beginning, were to manifest a growing and swarming specific multiplicity; it has an impact only on the advent of the popular myth of "Science" among the averate idolators of the goddess Reason and the god Progress, her divine Spouse and Handyman. But Descartes' theory warped philosophy. ON the one hand, raising human knowledge to a divine condition (for only in God is knowledge identical with the intellect and the essence of the knower; and just as for Leibnitz dum Deus calculat, fit mundus, so, for Descartes, while the human mind calculates the world is revealed in science), this theory was a first step in that progressive deification of man which came to completion in Hegel's philosophy. ON the other hand it boiled down in reality to a dismissal fo wisdom to the advantage of science, and foreshadowed the disintegration, bewilderment or complete subservience of metaphysics which was to take place in a relatively short space of time.

3. There is indeed a knowledge which is both essentially one and truly universal in the speculative or theoretical order. It is metaphysical wisdom. Its intelligible perspective, focused on esse, the act of existing, is that of the highest degree of abstraction or eidetic visualization, in which being is grasped as separate (or capable of separation) from matter, and concepts and definitions are formed in the light of the intuition of being insofar as being. Hence the essential unity of metaphisics. At the same time metaphysics is truly universal knowledge since it is as ample, in the order of speculative knowing, as being itself in the order of reality.

But there is a price paid for this universality, and we are more aware of this fact than the ancients were. Metaphysics appears to us in its genuine garment, which is not the rich, diamond-studded mantle of an empress of the world, but rather the simple and humble robe of a hermit who has chosen voluntary poverty. The superiority of metaphysical wisdom, at the peak of human natural knowledge, comes from the very fact that it is detached from worldly riches, and above them. I mean to say that it has given up any claim to explain the detail of phenomena and the infinite marvels of experience. And it has carried along the philosophy of nature in this sort of superior detachment, to the extend to which the philosophy of nature, at its proper level, is also wisdom and deals also with being, that is, being as mutable.

The universe of metaphysics is infinite in scope, as vast as the world, and much more, since metaphysics can attain to the prime cause of the world, to the Pure Act in its divine transcendance. But the universe of metaphysics is limited in inquisitory and explanatory competence, it leaves to science the conquest of the treasures of the earth and the laws of phenomena. Metaphysics contents itself with the knowledge of the supreme or primary causes and raisons d'être, and of the superior intelligibility which is to be found where some aspect or property of being insofar as it is in being can be grasped in things and deciphered in the obscure thickness of existence.

Thus it is that at the highest degree of natural or merely rational knowledge metaphysics exercizes an architectonic power of unification over the whole fabric of the mind. This power is both a power of illumination and inspiration and a power of order and organization. It is a power of illumination and inspiration, not with regard to the ways and principles of explanation peculiar to the other fields of knowledge, but with regard to the general soundness, the basic equipment and the fundamental attitude of the human mind which, in a given state of culture, puts into play the weays and principles in question. And it is a power of order and organization, which depends mainly on the capacity that metaphysical wosdom possesses to encompass itself an all the other degrees of knowledge through its reflexive or critical function. For the theory of knowledge is part of metaphysics, and it is wisdom's privilege reflexively or critically to elucidate and defend its own principles as well as the principles of the other spheres of knowledge and the various sciences, thus making them aware of their own genuine truth and competence and their own genuine frontiers.

In a universe of thought or a universe of culture at the summit of which the role to be played by metaphysical wisdom were recognized, there would exist, by this very fact, a certain common agreement on primary verities and on the primary principles of the workings of the mind, so that such a universe of thought or of culture, however diversified in its specialized scientific fields, would escape being basically divided against inself. And there would also exist a certain unity of intellectual inspiration, thanks to which all possible conflicts at the various levels of knowledge would develop without winding up in a final babelian chaos.

Yet it is the unity of order or organization of which I just spoke which is, to my mind, of most crucial imporatnce for the sciences. For there exist between the sciences and the great types of knowledge conflicts and oppositions which are stimulating and fertile because they proceed from natural tensions. But there are other conflicts and oppositions which are unnatural, and which hamper and warp the life of the mind because they proceed from misunderstanding and blindness. The roots of such unnatural conflicts and oppositions are on the one hand the ignorance in which the sciences and the great types of knowledge are (I mean in their representatives), not only of one another, but also of their own noetic nature, their own place in the hierarchy of knowing activities, and their own limits; and on the other hand the resulting claim made by each one to possess alone true methodological value and real power of explanation, and to deny to the others any right to exist. Today the place of metaphysics is being usurped by logical positivism, whose great discovery is that all assertions which are worthy of the intellect have no meaning at all. The only way to get rid of these evils is to recognize the proper field of competence and teh autonomy of our various approaches to reality, at the various stages of the universe of truth, and the unity of order of human knowledge.

As I observed in another study, human knowledge has unity only as an ensemble of specifically differentiated intellectual virtues, which extricate from sensory experience an intelligible content with specifically diverse powers of abstraction. "In the history of human knowledge we see now one, now another of these intellectual virtues, now one, now another of these types of knowledge, trying with a sort of imperialism, to seize, at the expense of the others, the whole universe of knowledge. Thus, at the time of Plato and Aristotle, there was a period of philosophical and metaphysical imperialism; in the Middle Ages, at least before Thomas Aquinas, a period of theological imperialism, since Descartes, Kant and August Comte, a period of scientific imperialism which has progressively lowered the level of reason while at the same time securing a splendid technical domination of material nature.

"It would be a great conquest if the huyman mind could end these attempts at spiritual imperialism which bring in their wake no less serious damage, to be sure, than that which results from political imperialism; it would be a great achievement if the human mind could establish on unshakable foundations the freedom and autonomy as well as the vital harmony and the mutual strengthening of the great disciplines of knowledge through which the intellect of man strives indefatigably toward truth." [The Range of Reason, p. 11].


4. Yet it is not enough to lay stress on the unity of order, as basic as it may be. There is another kind of unity, which is also indispensable, and which I would call unity of integration. For the various disciplines of knowledge are not only superordinate to one another, by reason of their "formal" approach, they are also in necessary intercommunication, to the extend to which they have to do with the same "material" set of reality or the same subject-matter, seen from typically distinct points of view. On the other hand, if the remarks put forward at the beginning of this paper are correct, it appears that in each case the unity of which we are now speaking is revealed at the level of that discipline of knowledge which is "supra- valent" or superordinate with respect to other ways of knowledge that are unified, not by commingling with one another, but by being integrated - above their own specific fields - to this superior discipline's own purposes, perspective and light.

A kind of unity of integration exists, thus, between the philosophy of art and the various arts as well as the various fields of art criticism, or between moral philosophy and inductive, factual disciplines like sociology or anthropology, - or between metaphysics and the philosophy of nature since mutable or material being is being, and therefore exemplifies in a particular and subordinate manner the laws of being qua being. But it is in the relationship between the philosophy of nature and the sciences of nature that we find the most significant and far-reaching instance of unity of integration.

I emphasized in the first part of this paper the specific distinction between the philosophy of nature and thes ciences of nature. Though they pertain to the same generic degree of abstraction - speaking of the degrees of abstraction, which have analogical, not univocal meaning, and involve typicall ydifferent approaches, I am bold enough to say, parenthetically, that Cajetan's and John of St. Thomas' classical interpretation can hardly be overthrown even by the Reverend Father Joseph T. Clark, though the philosophy of nature and the sciences of nature pertain to the same generic degree of abstraction (the first degree of abstraction, which is focused on a certain nature, material nature), by reason of the terminus a quo (the philosophy of nature and the sciences of nature similarly prescind from "individual matter"), they are, within this generic community, at specifically distinct levels of abstraction, by reason of the terminus ad quem, that is, of the positive approach to reality, which is ontological in the case of the philosophy of nature, empiriological in the case of the science; the ways in which the proper tools or instruments of knowing, the concepts and the definitions are formed and constructed are essentially different in both cses: here concepts and definitions are resolved in being, and thus participate in the superior intelligibility of metaphysics, there they are resolved in the observable and measurable as such, and provide the mind with entia rationis cum fundamento in re, thus participating or aspiring to participate in the superior intelligibility of mathematics. And similarly the light oin which the judgments are elicited is different in one case and in the other.

Nevertheless the philosophy of nature and the sciences of nature have to do with the same subject-matter, the world of nature, the material universe. And as a result, becasue they enlighten us, each one in its own way, on the same mysterious reality, not only are they, of necessity, complementary types of knowledge, but it is also necessary that the sciences of nature should on the one hand enter the very work and life of the philosophy of nature with a supply of knowledge of their own, and, on the other hand, be given by it some kind of interpretation in its own terms.

This is the unity of integration which we are discussing. And so, through an apparent paradox which is quite normal indeed, what is at an infra- valent level - the sciences of nature - depend to a lesser extent on what is at a supra-valent or superordinate level - the philosophy of nature - than the philosophy of nature depends on the sciences of nature: the dependence of the philosophy of nature with respect to the sciences of nature pertaining, moreover, to the material, not formal dependence, and being required by the very function of final integration which belongs to any superior discipline as such.

5. What, then, is the impact of physics and the natural sciences upon the philosophy of nature? Or, more precisely, in what way and according to what procedure does the philosophy of nature have to use the sciences of nature?

I should like first to observe that the notion of fact is an analogical notion, and that, accordingly, there are philosophical facts as well as scientific facts. A fact may be said to be a well ascertained existential truth. A truth is expressed in a judgment which links together two objects of concept. So a fact implies that a connection of objects exists a parte rei. In other words, what we call a fact implies the activity of the mind, - judgment. A fact implies discernment, an act of the mind. It implies simultaneously a judgment made by the mind and a perception, - in the case of the philosophy of nature and the sciences of nature, a sensory perception. "This being true, it is clear that there will be as many different degrees or orders of facts as there are orders of abstractive visualization for objective concepts. In other words, the very discernment of a fact takes place at a certain degree of abstraction. Facts are not all of equal rank, they are not all grouped at the same level in the market-place of sense experience so that the different sciences may come and pick out the wares they need. Facts themselves enter into the hierarchy of our knowledge. Wherefore there are common sense facts, scientific facts, by which I mean facts which are of interest to the natural sciences of phenomena, mathematical facts, metaphysical facts, etc. From these premises it follows that there are philosophical facts that are much more simple, much more general, much more evident and certain than the facts which are called scientific, i.e. the facts handled and interpreted by the natural sciences. For, as science progresses, these latter facts, especially in the physico-mathematical sciences, become points of contact of the real with increasingly complex constructions which have previously been set up by reason." There is, moreover, a philosophical critique of facts just as there is a scientific critique of facts. "And when a fact which is the result of absolutely general observation hs been judged and criticized by philosophy, it can no longer be called a fact of common observation, for the light of philosophical judgment and criticism has intervened to make it a philosophical fact in the strict sense of the word. The fact that something exists, that a multiplicity of things exists, that knowledge and thought exist, that becoming exists, these are all philosophical facts." [Philosophy of Nature, pp. 162-163].

Thus, the philosophy of nature deals with philosophical facts which pertain to its type of abstraction and conceptualization: for instance "change and becoming exist", "the continuous exists", "successive duration exists", "there are in animals mental processes somewhat similar to sense and memory processes in man", etc. And to the extend to which such philosophical facts can be established from common experience, philosophically criticized, the philosophy of nature is independend of the sciences of nature.

But the philosophy of nature does not and cannot deal only with such philosophical facts. Here we have that dpeendence of the philosophy of nature with respect to the sciences which I mentioned some moments ago. If it did not use also scientific facts for its own purposes, the philosophy of nature would remain intrinsically imperfect, and so to speak in an infantil state, - and it would fail in its duty of integration and unification.

Yet it is clear that the scientific facts as such are adapted to science, not to philosophy. They have been stated in the light of science, not of philosophy, and they are pregnant with scientific, not with philosophical conclusions.

How, then, can the philosophy of nature make use of them?

Philosophy cannot use them as "brute" scientific facts, I mean as data which have not been re-stated and re-interpreted by philosophy itself. This would wrong both science and philosophy, - inject in the statements of the former an ontological meaning with is not theirs, and impose on the latter the rule of notions which have only empriological bearing. We can neither claim to make the soul appear in a test tube nor claim that there is no soul because it does not appear in any test tube.

But my contention is that philosophy, while taking scientific facts in their truly and genuinely scientific meaning, can and must connect them with philosophical truths and data previously known at the proper level of philosophy; then it illumines them, so to speak, in the manner in which the intellect agent illumines the phantasms, and reveals in them a significance which up to that time was merely potential, and which transcends the plans of science. Thus we have scientific facts re-stated or re-interpreted by philosophy in its own light nd raised to the philosophical level. And then they are pregnant with philosophical conclusions because they have philosophical meaning, in other words because they have become philosophical facts. They are not philosophical facts from the start or by virtue of the impact of the Xrays of philosophical intelligence on common experience. They are philosophical facts in a final stage or by virtue of the impact of the Xrays which emanate from certain definite philosophical data upon scientifically elaborated experience.

The best way, I think, to make these things clear is to have recourse to some particular examples. The example I shall use today will deal with the question of hylomorphism. I should like to outline themethodological process through which, in my opinion, the philosophy of nature can make use in this case of scientific facts for its own philosophical purpose.

6. The first step will be a purely philosophical one, founded on originally philosophical facts, philosophical facts in the first sense I mentioned. The philosopher, starting for instance from an analysis of the abstractive function of the human mind, and of the fact that the object of the first apprehension is an intelligible quiddity which is universal in the mind and pertains to individual things in existential reality, will conclude that there are essences in the world of material nature. He can discern and circumscribe the most generic essences; perhaps he cannot, except in the case of man, discern and circumscribe the specific essences into which material beings are divided.

Then the philosopher, starting from the consideration of being as the primary object of the intellect, will establish the validity of the notion of substance. For things exist, and being is first of all that being which is fit to exist in itself, that is to say, substance. Otherwise no being would exist. The notion of accident appears in an indispensable but secondary way, when we turn our attention to change and becoming: being of being, that kind of being which is a determination of the substantial or root being of things, or which is fit to exist in another. The substance we have thus brought out, in contradistinction to the accident, is not substance in the watered-down sense of current language and the language of the sciences, - something observable which possesses stable marks of identification, - it is substance in the fully philosophical or ontological sense: the primary being through which a thing subsists and exists, and which is the root of its activity, and which, taken in itself, is a pure object of the intellect and does not appear to the senses.

Finally the philosopher, starting also from the consideration of being, but under the aspect of unity, will bring out the notion of substantial or primary unity, or in whit is unum per se, - in contradistinction to what is unum per accidens, or by reason of the bringing together of a multiplicity of substantial units, as is the case of a society, or also of an artificial whole like a machine.

These three data - there are specific essences, there is in the beings of nature a substance which is the invisible root of the accidents that appear to the senses, these beings are something unum per se, or endowed with substantial unity - are purely philosophical data. They are of no interest for the scientist, who is not concerned with them, because they are out of the range of his approach to nature.

Well, and this is the second step, once he is in possession of these three purely philosophical data, the philosopher turns toward the sciences of nature and their own data. He needs to do so, for he knows that there are specific essences, but he wonders what they are, or where they are, in what way they can be circumscribed. Science knows the infinitely variegated diversity of nature; will science teach the philosopher what those essences are which he is looking for? The misfortune is that scientific knowledge is not ontological knowledge. The species of the scientist are merely empiriological or taxonomic species, they are not the ontological species for which the philosopher is looking.

At this point the philosopheer is confronted with two possibilities. Either he will think with Mr. Mortimer Adler that the ontological species are very few in number, and coincide with those largest classes, - the realm of inanimate matter, the realm of vegetative life, the realm of animal life (not to speak of the realm of humanity), - whose intelligible characteristics or differences philosophical reason perceives. Or he will think that the ontological species, though they do not coincide with the taxonomic species of science and are doubtless larger, constitute a huge specific multiplicity within each of the above-mentioned realms.

7. Now comes the third step. If the philosopher subscribes to the positions of Mortimer Adler, then there is for him only one way to demonstrate hylomorphism. The fact chosen as starting point will be the change which occurs either in the case of the nutrition of a living organism, especially the human organism (whose substantial unity is certified by our consciousness of being a person), or in the case of its death and corruption.

And here the philosopher will not be satisfied with the observation of common experience, however obvious in itself, which enabled his colleagues of the Stone Age to be certain that there is an essential difference between a living and a non-living being. He will have to put to use all the data of the chemistry and biology of his time, and to analyze them in the light of the philosophical data he has established in his first step. He will have to deal with chemistry, as concerns the processes to which the materials of which food consists are subjected, and as concerns the composition of these materials which are complex structures involving a huge diversity of parts. And no doubt chemistry will not tell him what particles, in such complex structures, constitute the ontological units each of which is an individual substance. But be they atoms or molecules or constellations of molecules, it is enough for him to know that such ontological units exist, and that each of them is a substance one in itself, and that this substance, belonging to the realm of non-living matter, is distinct in essence from the organism.

On the other hand, the philosopher will have to deal with biology. Biology will tell him, for instance, that the organism elaborates within itself three kinds of materials: first, materials to be exploited for the production of inner energy; secondly, ferments or organic catalysts; thirdly, materials of construction, which are integrated in the structures of the living organism as such and are involved in its process of morphogenetic development. When it comes to this third category, one of the first scientific facts has to do with the existence, in the cell, of big molecules of protein that are endowed with self-catalytic properties by means of which they increase in volume. But this process of augmentation is not yet vital assimilation, any more than the process through which a crystal adds together the molecules which exist in its supersaturated solution. What characterizes vital assimilation is the fact, - here we have the crucial scientific fact, - that the foreign materials introduced into the cell are first broken down by it and then totally reconstructed by it, through the instrumentality of ferments, so as to acquire in their last detail the peculiar, and exceedingly diversified structure which is characteristic of the cell.

Such is the description given by the scientist. But the philosopher knows that the living organism, either unicellular or composed of a world of cells, is - from the very fact that the whole, in it, precedes the parts and forms them into existence - a single substance and an ontological unit unum per se. And he knows with full philosophical certitude that the essence of the substantial units which compose the food and the essence of the organism are specifically different, since the difference between a substance capable only of transitive activity and a substance endowed with immanent activity is a difference in the very essence of being.

As a result, he sees that the process of nutrition, considered in its ontological reality, consists in the production by the living organism, of a substance which is different in essence from the materials taken from the external environment, but which is but a new part of the same substance which is the individual substance of the organism. This is exactly what Aristotelian philosophy calls conversio alimenti intussuscepti in substantiam aliti. And this is evidently enough to conclude that such a change is a substantial change, and that it requires prime matter as its subject, and that, consequently, material beings are composed of prime matter and substantial form.

8. Here we have, in my opinion, the most convincing proof of hylomorphism. Assuming the position defended by Mortimer Adler as regards the species, it should even be considered the only proof. Now I must add that despite the remarkable discussion offered by Adler, and the extremely refined and impressive logic of his arguments, I do not feel persuaded of the necessity of his position. For various reasons, the principal of which is on the one hand the fecundity of being, on the other hand the divisive property which belongs to matter, I am inclined to believe that material nature is exceedingly rich in specific multiplicity - an indirect sign of which may be looked for in the discontinuity between the setes of unified characteristics that differentiate the great types of dynamic organization recongized by science.

Let us, then, consider this second possibility. Let us assume that the existence of a great diversity of ontological species in the material world has been established as a philosophical position which is not only likely, but certain. Then the proof of hylomorphism I just outlined will still remain valid, of course, and indeed the most valid proof. But it will cease to be the only one. The philosopher will also have to take into consideration other ways of establishing, perhaps with less demonstrative necessity, the philosophical fact of substantial change and the hylomorphic theory.

The questions which arise at this point are particularly difficult and subtle. For, as I just said, the ontological species are probably much fewer in number than the taxonomic ones. And even the most basic characteristics of the various types recognized by science are only empiriological signs deprived of cogent necessity.

I would say, nevertheless, that these fundamental taxonomic characteristics, - such as those which differentiate, in zoology, the insects from the vertebrates, and the great classes of vertebrates from one another, or those which in chemistry differentiate the elements of the periodical table, - such fundamental taxonomic characteristics, if they are related to a previously established certitude, founded on serious philosophical reasons, that there exists in the universe of matter a large variety of ontological species, take on the value of signs affording us with a high probability that the types in question are specifically distinct ontological species or essences. Leaving aside the problem of the compounds, the philosopher - supposing he has been able to establish with certainty, for merely philosophical reasons, the existence of a large multiplicity of ontological species in the realm of inanimate matter - may safely assume as highly probable that the empiriological classification of the table of Mendelejeff is a valid sign of an ontological, specific difference between the elements (the isotopes being regarded as varieties of the species). Then he will reason on these elements as on empiriological images or symbols of real essences which are specifically diverse. The factor of probability will have to do with the validity of the images or symbols, while the factor of certitude will have to do with the existence of a multiplicity of ontological species, whatever they may be. And, given such certitude, the philosopher will also be sure that if science, in a future stage of its development, should cling to a different picture of the basic empiriological diversities in the realm of matter, this picture will provide him with other images or symbols which will similarly be highly probable signs of a real ontological diversity. Today, in any case, he is entitled to use the images and symbols that are at a level with the state of science in our day. And thus he may use with a high degree of probability the elements of the periodical table as symbolic substitutes for ontological species which are specifically distinct, and state, therefore, that the atomic transmutations verified by modern science witness to real substantial changes.

Thus can the philosophy of nature integrate the data of chemistry in its own structure and for its own purposes, and at the same time establish the hylomorphist theory in a highly probable way, which is a valuable complement to the strictly demonstrative proof with which I was concerned in the previous section.

9. Now a fourth step, I think, must be taken. It is not enough for the philosophy of nature to have established the hylomorphic theory. It must, thereafter, point out what interpretation should be offered, from the philosophical point of view, of the theories of physicists about matter. How can the ontological notion and the physico-mathematical notion of matter fit together?

The primary thing is to realize that the expression "structure or constitution of matter" does not have the same meaning for the philosopher and for the physicist. To the philosopher it means the structure or constitution of matter qua substantial being, - to the physicist it means the structure or constitution of matter qua phenomenon, or insofar as a consistent representation of it can be worked out through our procedures of observation and measurement.

These two concepts of the constitution of matter are both valid, but not at the same level or on the same plane. It is obvious that we should neither try to fuse them with one another nor seek in the one the principles or foundation of the other.

When physics speaks of matter (or mass) and energy, and states that matter can be transformed into energy and conversely, it does not in any way have in view what the philosophy of nature calls the substance of material things, - this substance, considered in itself (prescinding from its accidents) is purely intelligible and cannot be graspedd by the senses or by any means of observation and measurement. The matter and energy or physics are physico-mathematical entities which are worked out by the mind in order to express reality, and which correspond symbolically to what the philosopher calls the proper accidents or structural properties of material substance (quantity and qualities). What we can say from the point of view of philosophy or ontological knowledge, is therefore that material substance, considered in such or such an element of the periodical table (then it is disclosed to us only symbolically, under the aspect of the "atom" of physics) possesses, by virtue of its proper accidents or structural properties, a certain organization in space (which is symbolically disclosed to us under the features of the system of electrons, protons, neutrons, etc. of physics) and a specific activity which derives from its very essence (and which is symbolically disclosed to us as the "energy" invested in the system in question). Then, when it comes the the case of atomic transmutation, the change which occurs in the system of electrons, for instance the loss of an electron through some kind of atomic bombardment, will be regarded by the philosopher as a symbolic image, in the field of physico-mathematical entities, of what constitutes ontologically the ultimate disposition of matter which determines the substantial change, at the instant when the previous substance is "corrupted", and the new substance "generated."

10. Finally, as concerns no longer the hylomorphic theory, but the general relationship between the philosophy of nature and the sciences of nature, another aspect of this relationship must be taken into consideration: it no longer relates to the integration of the scientific data in the proper work and structure of philosophical knowledge, but to the impact of philosophical knowledge on science itself. I do not mean to say that any philosophical or ontological notion may enter the field of scientific explanation. But when it comes to the whole concrete fabric of the mind of the scientist, it appears that a need and desire for a deeper elucidation of the mystery of nature, which ontological knowledge alone can afford, spontaneously and inevitably develops in the scientist, from his very experience in his own particular scientific field; thus he tends to work out some kind of philosophy of nature; and the philosophical (now and then pseudo-philosophical) notions to which he becomes attached have in turn a vital repercussion on the general trend of his intellect and imagination, and, consequently, on his process, both unconscious and conscious, of discovery and creative invention.

This is a sign of the fact that, in the normal organization of human knowledge, the philosophy of nature provides the sciences of nature, not with "constitutive principles", but, to use a Kantian distinction, with regulative principles, which orient thought and research, and play their part principally in that pre-rationalized, pre-systematized, pre- scientific backstage where the most decisive initial steps are taken.

The considerations I have submitted on the relationship between physics or chemistry and the philosophy of nature may be extended, mutatis mutandis, to the whole world of the sciences of nature, - including sciences like experimental psychology. I apologize for having dwelt on these problems at some length. It was an opportunity to make clearer (I hope) the positions I have propounded for a great many years, and which ahve sometimes been misunderstood by scholastic critics (I do not speak of the others) who are too much concerned, it seems to me, with literal but contingent aspects of the teaching of our common masters. But it was also required by the topic of this lecture. For we have in the particular problem in question a significant example of the way in which philosophy - and especially the philosophy of nature, whose realm is coextensive with that of all the sciences of nature together - is able to bring these sciences into unity, at the superordinate level of philosophy itself.