Part 3

The point to note is that Aristotle begins his investigation into the structure of What Is and the causes by which it is produced by starting from the existence of individual things belonging to the physical order and perceived by the senses. About any such thing we may ask two questions, (1) into what constituent factors can it be logically analysed? (2) and how has it come to exhibit the character which our analysis shows it to have? The answer to these questions will appear from a consideration of two standing antitheses which run through Aristotle's philosophy, the contrast between Matter and Form, and that between Potential and Actual, followed by a recapitulation of his doctrine of the Four Causes, or four senses of the word Cause.

*Matter and Form*.--Consider any completely developed individual thing, whether it is the product of human manufacture, as a copper bowl, or of natural reproduction, as an oak-tree or a horse. We shall see at once that the bowl is like other articles made of the same metal, candlesticks, coal-vases, in being made of the same stuff, and unlike them in having the special shape or structure which renders it fit for being used as a bowl and not for holding a candle or containing coals. So a botanist or a chemist will tell you that the constituent tissues of an oak or horse, or the chemical elements out of which these tissues are built up are of the same kind as those of an ash or an ox, but the oak differs from the ash or the horse from the ox in characteristic structure. We see thus that in any individual thing we can distinguish two components, the stuff of which it consists--which may be identical in kind with the stuff of which things of a very different kind consist--and the structural law of formation or arrangement which is peculiar to the "special" kind of thing under consideration. In the actual individual thing these two are inseparably united; they do not exist side by side, as chemists say the atoms of hydrogen and oxygen do in a drop of water; the law of organisation or structure is manifested in and through the copper, or the various tissues of the living body. Aristotle expresses this by saying that you can distinguish two aspects in an individual, its Matter, (hyle, materia) and its Form (eidos, forma). The individual is the matter as organised in accord with a determinate principle of structure, the form. Of these terms, the former, hyle (materia, matter) means literally timber, and more specifically ship's timbers, and his selection of it to mean what is most exactly rendered by our own word "stuff" may perhaps be due to a reminiscence of an old Pythagorean fancy which looked on the universe as a ship. The word for form is the same as Plato's, and its philosophical uses are closely connected with its mathematical sense, "regular figure," also a Pythagorean technicality which still survives in certain stereotyped phrases in Euclid. Aristotle extends the analysis into Matter and Form by analogy beyond the range of individual substances to everything in which we can distinguish a relatively indeterminate "somewhat" and a law or type of order and arrangement giving it determination. Thus if you consider the relatively fixed or "formed" character of a man in adult life, we may look upon this character as produced out of the "raw material" of tendencies and dispositions, which have received a specific development along definite lines, according to the kind of training to which the mind has been subjected in the "formative" period of its growth. We may therefore speak of native disposition as the matter or stuff of which character is made, and the practical problem of education is to devise a system of training which shall impress on this matter precisely the form required if the grown man is to be a good citizen of a good state. Since a man's character itself is not a substance but a complex of habits or fixed ways of reacting upon suggestions coming from the world around him, this is a good instance of the extension of the antithesis of Matter and Form beyond the category of substance. We see then that Matter in the Aristotelian sense must not be confounded with body; the relatively undetermined factor which receives completer determination by the structural law or Form is Matter, whether it is corporeal or not. This comes out with particular clearness in the metaphysical interpretation put on the logical process of definition by genus and difference. When I define any real kind by specifying a higher and wider class of which it is a sub-kind, and adding the peculiar characteristics which distinguish the sub-kind under consideration from the other sub-kinds of the same genus, the genus may be said to stand to the "differences" as Matter, the relatively indeterminate, to the Form which gives it its structure.

We further observe that Matter and Form are strictly correlative. The matter is called so relatively to the form which gives it further determination. When the words are used in their strictest sense, with reference to an individual thing, the Form is taken to mean the last determination by which the thing acquires its complete character, and the Matter is that which has yet to receive this last determination. Thus in the case of a copper globe, the spherical figure is said to be its Form, the copper its material. In the case of the human body, the Matter is the various tissues, muscles, bones, skin, &c. But each of these things which are counted as belonging to the Matter of the globe or the human body has, according to Aristotle, a development behind it. Copper is not an "element" but a specific combination of "elements," and the same thing is even more true of the highly elaborate tissues of the living body. Thus what is Matter relatively to the globe or living body is Matter already determined by Form if we consider it relatively to its own constituents. The so-called "elements" of Empedocles, earth, water, air, fire, are the matter of all chemical compounds, the Form of each compound being its specific law of composition; the immediate or "proximate" Matter of the tissues of the animal body is, according to Aristotle's biology, the "superfluous" blood of the female parent, out of which the various tissues in the offspring are developed, and the Matter of this blood is in turn the various substances which are taken into the body of the parent as food and converted by assimilation into blood. Their Matter, once more, is the earth, air, fire, and water of which they are composed. Thus at every stage of a process of manufacture or growth a fresh Form is superinduced on, or developed within, a Matter which is already itself a combination of Matter and Form relatively to the process by which it has itself been originated. Fully thought out, such a view would lead to the conclusion that in the end the simple ultimate matter of all individual things is one and the same throughout the universe, and has absolutely no definite structure at all. The introduction of Form or determinate structure of any kind would then have to be thought of as coming from an outside source, since structureless Matter cannot be supposed to give itself all sorts of specific determinations, as has been demonstrated in our own times by the collapse of the "Synthetic Philosophy." Aristotle avoids the difficulty by holding that "pure Matter" is a creation of our thought. In actual fact the crudest form in which matter is found is that of the "elements." Since the transmutability of the "elements" is an indispensable tenet in Aristotle's Physics, we cannot avoid regarding earth, water, fire air as themselves determinations by specific Form of a still simpler Matter, though this "prime Matter" "all alone, before a rag of Form is on," is never to be found existing in its simplicity.[#]

"He had First Matter seen undressed; He took her naked all alone, Before one rag of Form was on."

*The Potential and the Actual*.--So far we have been looking at the analysis of the individual thing, as the current jargon puts it, statically; we have arrived at the antithesis of Matter and Form by contrasting an unfinished condition of anything with its finished condition. But we may study the same contrast dynamically, with special reference to the process of making or growth by which the relatively undetermined or unfinished becomes determined or finished. The contrast of Matter with Form then passes into the contrast between Potentiality and Actuality. What this antithesis means we can best see from the case of the growth of a living organism. Consider the embryos of two animals, or the seeds of two plants. Even a botanist or a physiologist may be unable to pronounce with certainty on the species to which the germ submitted to him belongs, and chemical analysis may be equally at a loss. Even at a later stage of development, the embryo of one vertebrate animal may be indistinguishable from that of another. Yet it is certain that one of two originally indistinguishable germs will grow into an oak and the other into an elm, or one into a chimpanzee and the other into a man. However indistinguishable, they therefore may be said to have different latent tendencies or possibilities of development within them. Hence we may say of a given germ, "though this is not yet actually an oak, it is potentially an oak," meaning not merely that, if uninterfered with, it will in time be an oak, but also that by no interference can it be made to grow into an elm or a beech. So we may look upon all processes of production or development as processes by which what at first possessed only the tendency to grow along certain lines or to be worked up into a certain form, has become actually endowed with the character to which it possessed the tendency. The acorn becomes in process of time an actual oak, the baby an actual man, the copper is made into an actual vase, right education brings out into active exercise the special capacities of the learner. Hence the distinction between Matter and Form may also be expressed by saying that the Matter is the persistent underlying substratum in which the development of the Form takes place, or that the individual when finally determined by the Form is the Actuality of which the undeveloped Matter was the Potentiality. The process of conception, birth, and growth to maturity in Nature, or of the production of a finished article by the "arts" whose business it is to "imitate" Nature, may be said to be one of continuous advance towards the actual embodiment of a Form, or law of organisation, in a Matter having the latent potentiality of developing along those special lines. When Aristotle is speaking most strictly he distinguishes the process by which a Form is realised, which he calls Energeia, from the manifestation of the realised Form, calling the latter Entelechy (literally "finished" or "completed" condition). Often, however, he uses the word Energeia more loosely for the actual manifestation of the Form itself, and in this he is followed by the scholastic writers, who render Energeia by actus or actus purus.

One presupposition of this process must be specially noted. It is not an unending process of development of unrealised capacities, but always has an End in the perfectly simple sense of a last stage. We see this best in the case of growth. The acorn grows into the sapling and the sapling into the oak, but there is nothing related to the oak as the oak is to the sapling. The oak does not grow into something else. The process of development from potential to actual in this special case comes to an end with the emergence of the mature oak. In the organic world the end or last state is recognised by the fact that the organism can now exercise the power of reproducing its like. This tendency of organic process to culminate in a last stage of complete maturity is the key to the treatment of the problem of the "true end" of life in Aristotle's Ethics.

*The Four Causes*.--The conception of the world involved in these antitheses of Form and Matter, Potential and Actual, finds its fullest expression in Aristotle's doctrine of the Four Causes or conditions of the production of things. This doctrine is looked on by Aristotle as the final solution of the problem which had always been the central one for Greek philosophy, What are the causes of the world-order? All the previous philosophies he regards as inadequate attempts to formulate the answer to this question which is only given completely by his own system. Hence the doctrine requires to be stated with some fullness. We may best approach it by starting from the literal meaning of the Greek terms aitia, aition, which Aristotle uses to convey the notion of cause. Aition is properly an adjective used substantially, and means "that on which the legal responsibility for a given state of affairs can be laid." Similarly aitia, the substantive, means the "credit" for good or bad, the legal "responsibility," for an act. Now when we ask, "what is responsible for the fact that such and such a state of things now exists?" there are four partial answers which may be given, and each of these corresponds to one of the "causes." A complete answer requires the enumeration of them all. We may mention (1) the matter or material cause of the thing, (2) the law according to which it has grown or developed, the form or formal cause, (3) the agent with whose initial impulse the development began--the "starting-point of the process," or, as the later Aristotelians call it, the efficient cause, (4) the completed result of the whole process, which is present in the case of human manufacture as a preconceived idea determining the maker's whole method of handling his material, and in organic development in Nature as implied in and determining the successive stages of growth--the end or final cause. If any one of these had been different, the resultant state of things would also have been different. Hence all four must be specified in completely accounting for it. Obvious illustrations can be given from artificial products of human skill, but it seems clear that it was rather reflection on the biological process of reproduction and growth which originally suggested the analysis. Suppose we ask what was requisite in order that there should be now an oak on a given spot. There must have been (1) a germ from which the oak has grown, and this germ must have had the latent tendencies towards development which are characteristic of oaks. This is the material cause of the oak. (2) This germ must have followed a definite law of growth; it must have had a tendency to grow in the way characteristic of oaks and to develop the structure of an oak, not that of a plane or an ash. This is form or formal cause. (3) Also the germ of the oak did not come from nowhere; it grew on a parent oak. The parent oak and its acorn-bearing activity thus constitute the efficient cause of the present oak. (4) And there must be a final stage to which the whole process of growth is relative, in which the germ or sapling is no longer becoming but is an adult oak bearing fresh acorns. This is the end of the process. One would not be going far wrong in saying that Aristotle's biological cast of thought leads him to conceive of this "end" in the case of reproduction as a sub-conscious purpose, just as the workman's thought of the result to be attained by his action forms a conscious directing purpose in the case of manufacture. Both in Nature and in "art" the "form," the "efficient cause," and the "end" tend to coalesce. Thus in Nature "a man begets a man," organic beings give birth to other organic beings of the same kind, or, in the technical language of the Aristotelian theory of Causation, the efficient cause produces, as the "end" of its action, a second being having the same "form" as itself, though realised in different "matter," and numerically distinct from itself. Thus the efficient cause (i.e. the parent) is a "form" realised in matter, and the "end" is the same "form" realised in other matter. So in "products of art" the true "source of the process" is the "form" the realisation of which is the "end" or final cause, only with this difference, that as efficient cause the "form" exists not in the material but by way of "idea" or "representation" in the mind of the craftsman. A house does not produce another house, but the house as existing in "idea" in the builder's mind sets him at work building, and so produces a corresponding house in brick or stone. Thus the ultimate opposition is between the "cause as matter," a passive and inert substratum of change and development and the "formal" cause which, in the sense just explained, is one with both the "efficient" or starting-point, and the "end" or goal of development. It will, of course, be seen that individual bearers of "forms" are indispensable in the theory; hence the notion of activity is essential to the causal relation. It is a relation between things, not between events. Aristotle has no sense of the word cause corresponding to Mill's conception of a cause as an event which is the uniform precursor of another event.

Two more remarks may be made in this connection. (1) The prominence of the notion of "end" gives Aristotle's philosophy a thorough-going "ideological" character. God and Nature, he tells us, do nothing aimlessly. We should probably be mistaken if we took this to mean that "God and Nature" act everywhere with conscious design. The meaning is rather that every natural process has a last stage in which the "form" which was to begin with present in the agent or "source of change" is fully realised in the matter in which the agent has set up the process of change. The normal thing is e.g. for animals to reproduce "their kind"; if the reproduction is imperfect or distorted, as in monstrous births, this is an exception due to the occasional presence in "matter" of imperfections which hinder the course of development, and must be regarded as "contrary to the normal course of Nature." So hybrid reproduction is exceptional and "against Nature," and this is shown by the sterility of hybrids, a sort of lesser monstrosity. Even females, being "arrested developments," are a sort of still minor deviation from principle. (2) It may just be mentioned that Aristotle has a classification of efficient causes under the three heads of Nature, Intelligence (or Man), and Chance. The difference between Nature and Man or Intelligence as efficient causes has already been illustrated. It is that in causation by Nature, such as sexual reproduction, or the assimilation of nutriment, or the conversion of one element into another in which Aristotle believed, the form which is superinduced on the matter by the agent already exists in the agent itself as its form. The oak springs from a parent oak, the conversion of nutriment into organic tissue is due to the agency of already existing organic tissue. In the case of human intelligence or art, the "form" to be superinduced exists in the agent not as his characteristic form, but by way of representation, as a contemplated design. The man who builds a house is not himself a house; the form characteristic of a house is very different from that characteristic of a man, but it is present in contemplation to the builder before it is embodied in the actual house. A word may be added about the third sort of efficient causality, causation by chance. This is confined to cases which are exceptions from the general course of Nature, remarkable coincidences. It is what we may call "simulated purposiveness." When something in human affairs happens in a way which subserves the achievement of a result but was not really brought about by any intention to secure the result, we speak of it as a remarkable coincidence. Thus it would be a coincidence if a man should be held to ransom by brigands and his best friend should, without knowing anything of the matter, turn up on the spot with the means of ransoming him. The events could not have happened more opportunely if they had been planned, and yet they were not planned but merely fell out so: and since such a combination of circumstances simulating design is unusual, it is not proper to say that the events happened "in the course of Nature." We therefore say it happened by chance. This doctrine of chance has its significance for mediaeval Ethics. In an age when the Protestant superstition that worldly success is proof of nearness to God had not yet been invented, the want of correspondence between men's "deserts" and their prosperity was accounted for by the view that the distribution of worldly goods is, as a rule, the work of Fortune or Chance in the Aristotelian sense; that is, it is due to special coincidences which may look like deliberate design but are not really so. (See the elaborate exposition of this in Dante, Inferno, vii. 67-97.)

*Motion*.--We have seen that causation, natural or artificial, requires the production in a certain "matter" of a certain "form" under the influence of a certain "agent." What is the character of the process set up by the agent in the matter and culminating in the appearance of the form? Aristotle answers that it is Motion (kinesis). The effect of the agent on the matter is to set up in it a motion which ends in its assuming a definite form. The important point to be noted here is that Aristotle regards this motion as falling wholly within the matter which is to assume the form. It is not necessary that the agent should itself be in motion, but only that it should induce motion in something else. Thus in all cases of intentional action the ultimate efficient cause is the "idea of the result to be attained," but this idea does not move about. By its presence to the mind it sets something else (the members of the body) moving. This conception of an efficient cause which, not moving itself, by its mere presence induces movement in that to which it is present, is of the highest importance in Aristotle's theology. Of course it follows that since the motion by which the transition from potentiality to actuality is achieved falls wholly within the matter acted upon, Aristotle is not troubled with any of the questions as to the way in which motion can be transferred from one body to another which were so much agitated in the early days of the modern mechanical interpretation of natural processes. Aristotle's way of conceiving Nature is thoroughly non-mechanical, and approximates to what would now be called the ascription of vital or quasi-vital characteristics to the inorganic. As, in the causality of "art" the mere presence of the "form" to be embodied in a given material to the mind of the craftsman brings about and directs the process of manufacture, so in some analogous fashion the presence of an efficient cause in Nature to that on which it works is thought of as itself constituting the "efficiency" of the cause. As Lotze phrases it, things "take note of" one another's compresence in the universe, or we might say the efficient cause and that on which it exercises its efficiency are en rapport. "Matter" is sensitive to the presence of the "efficient cause," and in response to this sensitivity, puts forth successive determinations, expands its latent tendencies on definite lines.

The name "motion" has a wider sense for Aristotle than it has for ourselves. He includes under the one common name all the processes by which things come to be what they are or cease to be what they have been. Thus he distinguishes the following varieties of "motion": generation (the coming of an individual thing into being), with its opposite decay or corruption (the passing of a thing out of being), alteration (change of quality in a thing), augmentation and diminution (change in the magnitude of a thing), motion through space (of which latter he recognises two sub-species, rectilinear transference and rotation in a circular orbit about an axis). It is this last variety, motion through space, which is the most fundamental of all, since its occurrence is involved in that of any of the other types of process mentioned, though Aristotle does not hold the thorough-going mechanical view that the other processes are only apparent, and that, as we should put it, qualitative change is a mere disguise which mechanical motion wears for our senses.

*The Eternity of Motion*.--Certain very important consequences follow from the conception of efficient causation which we have been describing. Aristotle has no sympathy with the "evolutionist" views which had been favoured by some of his predecessors. According to his theory of organic generation, "it takes a man to beget a man "; where there is a baby, there must have been a father. Biological kinds representing real clefts in Nature, the process of the production of a young generation by an already adult generation must be thought of as without beginning and without end. There can be no natural "evolution" of animals of one species from individuals of a different kind. Nor does it occur to Aristotle to take into account the possibility of "Creationism," the sudden coming into being of a fully fledged first generation at a stroke. This possibility is excluded by the doctrine that the "matter" of a thing must exist beforehand as an indispensable condition of the production of that thing. Every baby, as we said, must have had a father, but that father must also have been a baby before he was a full-grown man. Hence the perpetuation of unchanging species must be without beginning and without end. And it is implied that all the various processes, within and without the organism, apart from which its life could not be kept up, must be equally without beginning and without end. The "cosmos," or orderly world of natural processes, is strictly "eternal"; "motion" is everlasting and continuous, or unbroken. Even the great Christian theologians who built upon Aristotle could not absolutely break with him on this point. St. Thomas, though obliged to admit that the world was actually created a few thousand years before his own time, maintains that this can only be known to be true from revelation, philosophically it is equably tenable that the world should have been "created from all eternity." And it is the general doctrine of scholasticism that the expression "creation" only denotes the absolute dependence of the world on God for its being. When we say "God created the world out of nothing," we mean that He did not make it out of pre-existing matter, that it depends for its being on Him only; the expression is purely negative in its import.

*God*.--With the doctrine of the eternity of the world and the processes which make up its life we come close to the culminating theory of Aristotelian First Philosophy, its doctrine of God, as the eternal, unchanging source of all change, movement, and process. All motion is a process within matter by which the forms latent in it are brought into actual manifestation. And the process only takes place in the presence of an adequate efficient cause or source of motion. Hence the eternity of natural processes involves the existence of one or more eternal sources of motion. For, if we do not admit the existence of an unoriginated and ever-present source or sources of motion, our only alternative is to hold that the world-process is due to a series of sources of motion existing successively. But such a view would leave the unity and unbroken continuity of the world-process unaccounted for. It would give us a succession of processes, temporally contiguous, not one unbroken process. Hence we argue from the continuity of motion to its dependence on a source or sources which are permanent and present throughout the whole everlasting world-process. And when we come to the question whether there is only one such ultimate source of movement for the whole universe, or several, Aristotle's answer is that the supreme "Unmoved Mover" is one. One is enough for the purpose, and the law of parcimony forbids us to assume the superfluous. This then is the Aristotelian conception of God and God's relation to the world. God is the one supreme unchanging being to whose presence the world responds with the whole process of cosmic development, the ultimate educer of the series of "forms" latent in the "matter" of the world into actual manifestation. Standing, as He does, outside the whole process which by His mere presence He initiates in Nature, He is not himself a composite of "form" and "matter," as the products of development are. He is a pure individual "form" or "actuality," with no history of gradual development behind it. Thus He is a purely immaterial being, indispensable to the world's existence but transcending it and standing outside it. How His presence inspires the world to move Aristotle tries to explain by the metaphor of appetition. Just as the good I desire and conceive, without itself "moving" "moves" my appetition, so God moves the universe by being its good. This directly brings about a uniform unbroken rotation of the whole universe round its axis (in fact, the alternation of day and night). And since this rotation is communicated from the outermost "sphere" of heaven to all the lesser "spheres" between it and the immovable centre, the effects of God's presence are felt universally. At the same time, we must note that though God is the supreme Mover of the Universe, He is not regarded by Aristotle as its Creator, even in the sense in which creation can be reconciled with the eternity of the world. For the effect of God's presence is simply to lead to the development of "form" in an already existing "matter." Without God there could be no "form" or order in things, not even as much as is implied in the differentiation of matter into the four "elements," yet "primary matter" is no less than God a precondition of all that happens.

It is characteristic of Aristotle that his God is as far from discharging the functions of a Providence as He is from being a Creator. His "activity" is not, as Plato had made it, that of the great "Shepherd of the sheep." As far as the world is concerned, God's only function is to be there to move its appetition. For the rest, the unbroken activity of this life is directed wholly inward. Aristotle expressly calls it an "activity of immobility." More precisely, he tells us, it is activity of thought, exercised unbrokenly and everlastingly upon the only object adequate to exercise God's contemplation, Himself. His life is one of everlasting self-contemplation or "thinking of thought itself." Like all unimpeded exercise of activity, it is attended by pleasure, and as the activity is continuous, so the pleasure of it is continuous too. At our best, when we give ourselves up to the pure contemplative activity of scientific thought or aesthetic appreciation, we enter for a while into this divine life and share the happiness of God. But that is a theme for our chapter on the Ethics.

It is a far cry from this conception of a God untroubled by care for a world to which He is only related as the object of its aspiration to the God who cares even for the fall of the sparrow and of whom it is written, Sic Deus dilexit mundum, but it was the standing task of the philosophical theologians of the Middle Ages to fuse the two conceptions. Plato's God, who, if not quite the Creator, is the "Father and Fashioner" of us all, and keeps providential watch over the world He has fashioned, would have lent Himself better to their purposes, but Plato was held by the mediaeval church to have denied the resurrection of the body. The combination of Aristotle's Theism with the Theism of early Christianity was effected by exquisitely subtle logical devices, but even in St. Thomas one cannot help seeing the seams.

Nor can one help seeing in Aristotle's own doctrine the usual want of coherence between an initial anti-Platonic bias and a final reversion to the very Platonic positions Aristotle is fond of impugning. We are told at the outset that the Platonic "separate forms" are empty names, and that the real individual thing is always a composite of matter and a form which only exists "in matter." We find in the end that the source of the whole process by which "matter" becomes imbued with "form" is a being which is "pure" form and stands outside the whole development which its presence sets up. And the issue of Aristotle's warning against "poetic metaphors" is the doctrine that God moves the world by being "the object of the world's desire."

*CHAPTER IV*

*PHYSICS*

There is no part of Aristotle's system which has been more carefully thought out than his Physics; at the same time it is almost wholly on account of his physical doctrines that his long ascendancy over thought is so much to be regretted. Aristotle's qualifications as a man of science have been much overrated. In one department, that of descriptive natural history, he shows himself a master of minute and careful observation who could obtain unqualified praise from so great a naturalist as Darwin. But in Astronomy and Physics proper his inferiority in mathematical thinking and his dislike for mechanical ways of explaining facts put him at a great disadvantage, as compared with Plato and Plato's Pythagorean friends. Thus his authority was for centuries one of the chief influences which prevented the development of Astronomy on right lines. Plato had himself both taught the mobility of the earth and denied correctly that the earth is at the centre of the universe, and the "Copernican" hypothesis in Astronomy probably originated in the Academy. Aristotle, however, insists on the central position of the earth, and violently attacks Plato for believing in its motion. It is equally serious that he insists on treating the so-called "four elements" as ultimately unanalysable forms of matter, though Plato had not only observed that so far from being the ABC (stoicheia or elementa, literally, letters of the alphabet) of Nature they do not deserve to be called even "syllables," but had also definitely put forward the view that it is the geometrical structure of the "corpuscles" of body upon which sensible qualities depend. It is on this doctrine, of course, that all mathematical physics rests. Aristotle reverts to the older theory that the differences between one "element" and another are qualitative differences of a sensible kind. Even in the biological sciences Aristotle shows an unfortunate proneness to disregard established fact when it conflicts with the theories for which he has a personal liking. Thus, though the importance of the brain as the central organ of the sensori-motor system had been discovered in the late sixth or early fifth century by the physician Alemacon of Crotona, and taught by the great Hippocrates in the fifth and by Plato in the fourth century, Aristotle's prejudices in favour of the doctrines of a different school of biologists led him to revert to the view that it is the heart which is the centre of what we now call the "nervous system." It is mainly on account of these reactionary scientific views that he was attacked in the early seventeenth century by writers like our own Francis Bacon, who found in veneration for Aristotle one of the chief hindrances to the free development of natural science. The same complaints had been made long before by critics belonging to the Platonic Academy. It is a Platonist of the time of Marcus Aurelius who sums up a vigorous attack on the Aristotelian astronomy by the remark that Aristotle never understood that the true task of the physicist is not to prescribe laws to Nature, but to learn from observation of the facts what the laws followed by Nature are.

In determining the scope of Physics, we have to begin by considering what is the special characteristic of things produced by Nature as contrasted with those produced by "art." The obvious distinction, intimated by the very etymology of the word "Nature" (physis, connected with phyesthai, to grow, to be born, as natura is with nasci), is that "what is by Nature" is born and grows, whereas what is as a result of artifice is made. The "natural" may thus be said to consist of living bodies and of their constituent parts. Hence inorganic matter also is included in "Nature," on the ground that living tissue can be analysed back into compounds of the "elements." Now things which are alive and grow are distinguished from things which are made by "a source of motion and quiescence within themselves"; all of them exhibit motions, changes of quality, processes of growth and decline which are initiated from within. Hence Nature may be defined as the totality of things which have a source of motion internal to themselves and of the constituent parts of such things. Nature then comprises all beings capable of spontaneous change. Whatever either does not change at all, or only changes in consequence of external influences, is excluded from Nature.

Thus the fundamental fact everywhere present in Nature is "change," "process," "motion." Since motion in the literal sense of change of position is involved as a condition of every such process, and such motion requires space through which to move and time to move in, the doctrine of space and time will also form part of Physics. Hence a great part of Aristotle's special lectures on Physics is occupied with discussion of the nature of space and time, and of the continuity which we must ascribe to them if the "continuous motion" on which the unbroken life of the universe depends is to be real Aristotle knows nothing of the modern questions whether space and time are "real" or only "phenomenal," whether they are "objective" or "subjective." Just as he simply assumes that bodies are things that really exist, whether we happen to perceive them or not, so he assumes that the space and time in which they move are real features of a world that does not depend for its existence on our perceiving it.

His treatment of space is singularly naif. He conceives it as a sort of vessel, into which you can pour different liquids. Just as the same pot may hold first wine and then water, so, if you can say, "there was water here, but now there is air here," this implies the existence of a receptacle which once held the water, but now holds the air. Hence a jug or pot may be called a "place that can be carried about," and space or place may be called "an immovable vessel." Hence the "place" of a thing may be defined as the boundary, or inner surface, of the body which immediately surrounds the thing. It follows from this that there can be no empty space. In the last resort, "absolute space" is the actual surface of the outermost "heaven" which contains everything else in itself but is not contained in any remoter body. Thus all things whatever are "in" this "heaven." But it is not itself "in" anything else. In accord with the standing Greek identification of determinate character with limitation, Aristotle holds that this outermost heaven must be at a limited distance from us. Actual space is thus finite in the sense that the volume of the universe could be expressed as a finite number of cubic miles or yards, though, since it must be "continuous," it is infinitely divisible. However often you subdivide a length, an area, or a volume, you will always be dividing it into lesser lengths, &c., which can once more be divided. You will never by division come to "points," i.e. mere positions without magnitude of divisibility.

The treatment of time is more thoughtful. Time is inseparably connected with movement or change. We only perceive that time has elapsed when we perceive that change has occurred. But time is not the same as change. For change is of different and incommensurate kinds, change of place, change of colour, &c.; but to take up time is common to all these forms of process. And time is not the same as motion. For there are different rates of speed, but the very fact that we can compare these different velocities implies that there are not different velocities of time. Time then is that in terms of which we measure motion, "the number of motion in respect of before and after," i.e. it is that by which we estimate the duration of processes. Thus e.g. when we speak of two minutes, two days, two months as required for a certain process to be completed, we are counting something. This something is time. It does not seem to occur to Aristotle that this definition implies that there are indivisible bits of time, though he quite correctly states the incompatible proposition that time is "made up of successive nows," i.e. moments which have no duration at all, and can no more be counted than the points on a straight line. He recognises of course that the "continuity" of motion implies that of time as well as of space. Since, however, "continuity" in his language means the same thing as indefinite divisibility, it ought not to be possible for him to regard time as "made up of nows"; time, like linear extension, ought for him to be a "length of" something.

*The Continuous Motion and the "Spheres."*--The continuous world-process depends upon a continuous movement set up in the universe as a whole by the presence of an everlasting and unchangeable "First Mover," God. From the self-sameness of God, it follows that this most universal of movements must be absolutely uniform. Of what precise kind can such a movement be? As the source of the movement is one, and the object moved is also one--viz. the compass of the "heaven," the movement of the primum mobile or "first moved"--the object immediately stimulated to motion by God's presence to it, must be mechanically simple. Now Aristotle, mistakenly, held that there are two forms of movement which are simple and unanalysable, motion of translation along a straight line, and motion of rotation round an axis. He is at pains to argue that rectilinear motion, which we easily discover to be that characteristic of bodies near the earth's surface when left to themselves, cannot be the kind of movement which belongs to the "heaven" as a whole. For continuous rectilinear movement in the same direction could not go on for ever on his assumption that there is no space outside the "heaven," which is itself at a finite distance from us. And motion to and fro would not be unbroken, since Aristotle argues that every time a moving body reached the end of its path, and the sense of its movement was reversed, it would be for two consecutive moments in the same place, and therefore at rest. Reversal of sense would imply a discontinuity. Hence he decides that the primary unbroken movement must be the rotation of the "first moved"--that is, the heaven containing the fixed stars--round its axis. This is the only movement which could go on for ever at a uniform rate and in the same sense. Starting with the conviction that the earth is at rest in the centre of the universe, he inevitably accounts for the alternation of day and night as the effect of such a revolution of the whole universe round an axis passing through the centre of the earth. The universe is thus thought of as bounded by a spherical surface, on the concave side of which are the fixed stars, which are therefore one and all at the same distance from us. This sphere, under the immediate influence of God, revolves on its axis once in twenty-four hours, and this period of revolution is absolutely uniform. Next the apparently irregular paths of the "planets" known to Aristotle (i.e. the moon, Mercury, Venus, the sun, Mars, Jupiter, Saturn) are resolved into combinations of similar uniform rotations, each planet having as many "spheres" assigned to it as are requisite for the analysis of its apparent path into perfectly circular elementary motions. Altogether Aristotle holds that fifty-four such rotating spheres are required over and above the "first moved" itself, whose rotation is, of course, communicated to all the lesser "spheres" included within it. As in the case of the "first moved," the uniform unceasing rotation of each "sphere" is explained by the influence on it of an unchanging immaterial "form," which is to its own "sphere" what God is to the universe as a whole. In the Aristotelianism of the mediaeval church these pure forms or intelligences which originate the movements of the various planetary spheres are naturally identified with angels. It is e.g. to the angelic intelligences which "move" the heaven of Venus, which comes third in order counting outward from the earth, that Dante addresses his famous Canzone, Voi ch' intendendo il terzo del movete. The mediaeval astronomy, however, differs in two important respects from that of Aristotle himself. (1) The number of "spheres" is different. Increasing knowledge of the complexity of the paths of the planets showed that if their paths are to be analysed into combinations of circular motions, fifty-four such rotations must be an altogether inadequate number. Aristotle's method of analysis of the heavenly movements was therefore combined with either or both of two others originated by pure astronomers who sat loose to metaphysics. One of these methods was to account for a planet's path by the introduction of epicycles. The planet was thought of not as fixed at a given point on its principal sphere, but as situated on the circumference of a lesser sphere which has its centre at a fixed point of the principal sphere and rotates around an axis passing through this centre. If need were, this type of hypothesis could be further complicated by imagining any number of such epicycles within epicycles. The other method was the employment of "eccentrics," i.e. circular movements which are described not about the common centre of the earth and the universe, but about some point in its neighbourhood. By combinations of epicycles and eccentrics the mediaeval astronomers contrived to reduce the number of principal spheres to one for each planet, the arrangement we find in Dante. (2) Also real or supposed astronomical perturbations unknown to Aristotle led some mediaeval theorists to follow the scheme devised by Alphonso the Wise of Castille, in which further spheres are inserted between that of Saturn, the outermost planet, and the "first moved." In Dante, we have, excluding the "empyrean" or immovable heaven where God and the blessed are, nine "spheres," one for each of the planets, one for the fixed stars, and one for the "first moved," which is now distinguished from the heaven of the stars. In Milton, who adopts the "Alphonsine" scheme, we have further a sphere called the "second movable" or "crystalline" introduced between the heaven of the fixed stars and the "first moved," to account for the imaginary phenomenon of "trepidation."[#] In reading Dante, Shakespeare, and Milton, we have always to remember that none of these reproduces the Aristotelian doctrine of the "spheres" accurately; their astronomy is an amalgam of Aristotle, Ptolemy, and Hipparchus.

"They pass the planets seven, and pass the fixed, And that crystalline sphere whose balance weighs The trepidation talked, and that first moved."

So far, the doctrine of the fifty-five "spheres" might be no more than a legitimate mathematical fiction, a convenient device for analysing the complicated apparent movements of the heavenly bodies into circular components. This was originally the part played by "spheres" in ancient astronomical theory, and it is worth while to be quite clear about the fact, as there is a mistaken impression widely current to-day that Aristotle's astronomy is typical of Greek views in general. The truth is that it is peculiar to himself. The origin of the theory was Academic. Plato proposed to the Academy as a subject of inquiry, to devise such a mathematical analysis of astronomical motions as will best "save the appearances," i.e. will most simply account for the apparent paths of the planets. The analysis of these paths into resultants of several rotations was offered as a solution by the astronomer Eudoxus of Cnidus. So far, the "spheres," then, were a mere mathematical hypothesis. What Aristotle did, and it is perhaps the most retrograde step ever taken in the history of a science, was to convert the mathematical hypothesis into physical fact. The "spheres" become with him real bodies, and as none of the bodies we are familiar with exhibit any tendency to rotate in circles when left to themselves, Aristotle was forced to introduce into Physics the disastrous theory, which it was a great part of Galileo's life-work to destroy, that the stuff of which the spheres are made is a "fifth body," different from the "elements" of which the bodies among which we live are made. Hence he makes an absolute distinction between two kinds of matter, "celestial matter," the "fifth body," and "terrestrial" or "elementary" matter. The fundamental difference is that "terrestrial" or "elementary" matter, left to itself, follows a rectilinear path, "celestial" matter rotates, but it is further inferred from the supposed absolute uniformity of the celestial movements that "celestial matter" is simple, uncompounded, incapable of change, and consequently that no new state of things can ever arise in the heavens. The spheres and planets have always been and will always be exactly as they are at the present moment. Mutability is confined to the region of "terrestrial" or "elementary" matter, which only extends as far as the orbit of the moon, the "lowest of the celestial bodies," because it is only "terrestrial" things which are, as we should say, chemical compounds. This is the doctrine which Galileo has in mind when he dwells on such newly-discovered astronomical facts as the existence of sun-spots and variable stars, and the signs of irregularity presented by the moon's surface. The distinction is peculiar to Aristotle. No one before him had ever thought of supposing the heavenly bodies to be made of any materials other than those of which "bodies terrestrial" are made. In the Academic attack on Aristotle's science of which we have already spoken the two points singled out for reprobation are (1) his rejection of the principle that all moving bodies, left to themselves, follow a rectilinear path, and (2) his denial that the heavenly bodies are made of the same "elements" as everything else. (It may just be mentioned in passing that our word quintessence gets its sense from the supposed special "nobility" of the incorruptible "fifth body.")

*Terrestrial Bodies*.--As we have seen already, Aristotle was out of sympathy with the tendency to regard the sensible differences between bodies as consequences of more ultimate differences in the geometrical structure of their particles. Hence his whole attitude towards the problems of that branch of natural science which we call physics is quite unlike any view to which we are accustomed. He reverts from the mathematical lines of thought current in Plato's Academy to the type of view more natural to the "plain man," and, like the earliest sixth-century men of science, regards the qualitative differences which our senses apprehend as fundamental. Among these, particular stress is laid on the difference in sensible temperature (the hot--the cold), in saturation (the dry--the moist), and in density (the dense--the rare). If we consider the first two of these oppositions, we can make four binary combinations of the elementary "opposite" characters, viz. hot and dry, hot and moist, cold and moist, cold and dry. These combinations are regarded as corresponding respectively to the sensible characteristics of the four bodies which Empedocles, the father of Greek chemistry, had treated as the ultimate components of everything. Fire is hot and dry, air hot and moist, water moist and cold, earth cold and dry. This reflection shows us why Aristotle held that the most rudimentary form in which "matter" ever actually exists is that of one of these "elements." Each of them has one quality in common with another, and it is in virtue of this that a portion of one element can be assimilated by and transmuted into another, a process which seems to the untutored eye to be constantly recurring in Nature. We also observe that the order in which the "elements" appear, when so arranged as to form a series in which each term has one quality in common with each of its neighbours, is also that of their increasing density. This would help to make the conception of their transmutability all the more natural, as it suggests that the process may be effected by steady condensation. We must remember carefully that for Aristotle, who denies the possibility of a vacuum, as for the mediaeval alchemists, condensation does not mean a mere diminution of the distances between corpuscles which remain unchanged in character, but is a process of real qualitative change in the body which undergoes it. Incidentally we may remark that all changes of quality are regarded by Aristotle as stages in a continuous "movement" from one extreme of a scale to another. For example, colours, with him as with Goethe, form a series of which the "opposites" white and black are the end-points. Every other colour is a combination of white and black according to a definite proportion.

The Aristotelian doctrine of weight was one of the chief obstacles which seventeenth-century science had to contend with in establishing correct notions in dynamics. It is a curious feature of Greek science before Aristotle that, though the facts connected with gravity were well known, no one introduced the notion of weight to account for them. The difference between heavy bodies and light bodies had been previously treated as secondary for science. Plato's treatment of the matter is typical of the best fourth-century science. We must not try to explain why the heavier bodies tend to move towards the earth's surface by saying that they have a "downward" motion; their motion is not downward but "towards the centre" (the earth, though not fixed at the centre of the universe, being nearer to it than the rest of the solar and sidereal system). Plato then explains the tendency in virtue of which the heavier bodies move towards the "centre" as an attraction of like for like. The universal tendency is for smaller masses of "earth," "water," "air," "fire" to be attracted towards the great aggregations of the same materials. This is far from being a satisfactory theory in the light of facts which were not yet known to Plato, but it is on the right lines. It starts from the conception of the facts of gravity as due to an "attractive force" of some kind, and it has the great merit of bringing the "sinking" of stones and the "rising" of vapours under the same explanation.

Aristotle, though retaining the central idea that a body tends to move towards the region where the great cosmic mass of the same kind is congregated, introduced the entirely incompatible notion of an absolute distinction of "up" and "down." He identified the centre of the universe with that of the earth, and looked on motion to this centre as "downward." This led him to make a distinction between "heavy" bodies, which naturally tend to move "down," and "light" bodies, which tend to move "up" away from the centre. The doctrine works out thus. The heaviest elements tend to be massed together nearest the centre, the lightest to be furthest from it. Each element thus has its "proper place," that of water being immediately above earth, that of air next, and that of fire furthest from the centre, and nearest to the regions occupied by "celestial matter." (Readers of Dante will recollect the ascent from the Earthly Paradise through the "sphere of fire" with which the Paradiso opens.)