It is usually believed that Aristotle contradicted himself in as much as he first states, as above, that the individual object, the compound of universal and particular, is substance, but later on allows a superior reality to the universal, or "form" as he calls it, and in effect teaches, like Plato, that the universal is what alone is absolutely real, that is, that the universal is substance. I do not agree that there is any real inconsistency in Aristotle. Or rather, the inconsistency is one of words and not of thought. It must be remembered that, whenever Aristotle says that the individual, and not the universal, is substance, he is thinking of Plato. What he means to deny is that the universal can exist on its own account, as Plato thought. Nevertheless he agrees with Plato that the universal is the real. When he says that the universal is not substance he means, as against Plato, that it is not existent. What alone exists is the individual thing, the compound of universal and particular. When he says, or implies, that the universal is substance, he means that, though it is not existent, it is real. His words are contradictory, but his meaning is not. He has not expressed himself as clearly as he should; that is all.
The further development of Aristotle's metaphysics depends upon his doctrine of causation. By causation here, however, is meant a very much wider conception [{268}] than what is understood by that term in modern times. I have in previous lectures attempted to make clear the distinction between causes and reasons. The cause of a thing does not give any reason for it, and therefore does not explain it. The cause is merely the mechanism by which a reason produces its consequence. Death is caused by accident or disease, but these causes explain nothing as to why death should be in the world at all. Now if we accept this distinction, we may say that Aristotle's conception of causation includes both what we have called causes and reasons. Whatever is necessary, whether facts or principles, whether causes or reasons, fully to understand the existence of a thing, or the happening of an event, is included in the Aristotelian notion of causation.
Taking causation in this wide sense, Aristotle finds that there are four kinds of causes, the material, the efficient, the formal, and the final cause. These are not alternative causes; it is not meant that, to explain anything, one or other of the four must be present. In every case of the existence or production of a thing all four causes operate simultaneously. Moreover the same four causes are to be found both in human and in cosmic production, in the making of manufactured articles by man and in the production of things by nature. They are more clearly and easily seen, however, in human production, from which sphere, therefore, we select our example. The material cause of a thing is the matter of which it is composed. It is the raw material which becomes the thing. For example, in the making of a bronze statue of Hermes, the bronze is the material cause of the statue. This example might lead one to suppose [{269}] that Aristotle means by material cause what we call matter, physical substance, such as brass, iron, or wood. As we shall see later, this is not necessarily the case, though it is so in the present instance. The efficient cause is always defined by Aristotle as the cause of motion. It is the energy or moving force required to bring about change. It must be remembered that by motion Aristotle means not merely change of place but change of any sort. The alteration of a leaf from green to yellow is just as much motion, in his sense, as the falling of a stone. The efficient cause, then, is the cause of all change. In the example taken, what causes the bronze to become a statue, what produces this change, is the sculptor. He is, therefore, the efficient cause of the statue. The formal cause Aristotle defines as the substance and essence of the thing. Now the essence of a thing is given in its definition. But the definition is the explication of the concept. Therefore the formal cause is the concept, or, as Plato would call it, the Idea of the thing. Plato's Ideas thus reappear in Aristotle as formal causes. The final cause is the end, purpose, or aim, towards which the movement is directed. When a statue is being produced, the end of this activity, what the sculptor aims at, is the completed statue itself. And the final cause of a thing in general is the thing itself, the completed being of the object.
We can see at once how much wider this conception of causation is than the modern conception. If we take Mill's definition of a cause as the best expression of modern scientific ideas, we find that he defines a cause as the "invariable and unconditional antecedent of a phenomenon." This cuts out final causes at once. For [{270}] the final cause is the end, and is not an antecedent in time. It also does not include formal causes. For we do not now think of the concept of a thing as being part of its cause. This leaves us with only material and efficient causes, and these correspond roughly to the modern notions of matter and energy. Even the efficient causes of Aristotle, however, appear on further consideration, to be excluded from the modern idea of causation. For, though the efficient cause is the energy which produces motion, modern science regards it as purely mechanical energy, whereas Aristotle thinks of it, as we shall see, as an ideal force, operating not from the beginning but from the end. But it must not be supposed that, in saying that the modern idea of causation excludes formal and final causes, we mean that Aristotle is wrong in adding them, or that the modern idea is better than Aristotle's. It is not a question of better and worse at all. Modern science does not in any way deny the reality of formal and final causes. It merely considers them to be outside its sphere. It is no business of science whether they exist or not. As knowledge advances, differentiation and division of labour occur. Science takes as its province mechanical causes, and leaves formal and final causes to the philosopher to explicate. Thus, for example, formal causes are not considered by science because they are not, in the modern sense, causes at all. They are what we have called reasons. If we are to explain the existence of an object in the universe it may be necessary to introduce formal causes, concepts, to show why the thing exists, to show in fact its reasons. But science makes no attempt to explain the existence of objects. It takes their [{271}] existence for granted, and seeks to trace their history and their relations to each other. Therefore it does not require formal causes. It seeks to work out the mechanical view of the universe, and therefore considers only mechanical causes. But Aristotle's theory, as being philosophy rather than science, includes both the principles of mechanism and teleology.
It was not Aristotle's habit to propound his theories as if they were something absolutely new, sprung for the first time out of his own brain. In attacking any problem, his custom was to begin by enumerating current and past opinions, to criticise them, to reject what was valueless in them, to retain the residue of truth, and to add to it his own suggestions and original ideas. The resultant of this process was his own theory, which he thus represented, not as absolutely new, but as a development of the views of his predecessors. This course he follows also in the present instance. The first book of the "Metaphysics" is a history of all previous philosophy, from Thales to Plato, undertaken with the object of investigating how far the four causes had been recognized by his predecessors. The material cause, he says, had been recognized from the first. The Ionics believed in this and no other cause. They sought to explain everything by matter, though they differed among themselves as to the nature of the material cause, Thales describing it as water, Anaximenes as air. Later philosophers also gave different accounts of it, Heracleitus thinking it was fire, Empedocles the four elements, Anaxagoras an indefinite number of kinds of matter. But the point is that they all recognized the necessity for a material cause of some sort to explain the universe.
The earliest thinkers, then, the Ionics, assumed only this one cause. But as thought advanced, says Aristotle, and other philosophers came upon the scene, "the thing itself guided them." It was seen that a second cause was necessary to explain the motion and becoming of things. For matter itself does not produce its motion. Wood is not the cause of its becoming a bed, nor is brass the cause of its becoming a statue. Hence arose the idea of the efficient cause. The Eleatics did not recognize it, for they denied motion, and for them, therefore, no cause of motion could be assumed. But Parmenides, Aristotle thinks, wavered on this point, somehow allowing vaguely the existence of a second cause, which he denominated the hot and the cold. The reference is, of course, to the second part of the poem of Parmenides. Other philosophers clearly assumed an efficient cause, for they thought that one element, for example, fire, is more active, that is, more productive of motion, than others. Empedocles certainly attained to the idea of an efficient cause, for he named as moving forces, harmony and discord, love and hate. Anaxagoras also, used Nous as a moving force.
Formal causes had, perhaps, been recognized by the Pythagoreans, for numbers are forms. But they straightway degraded the formal cause to the level of a material cause by declaring that number is the stuff or matter of which things are made. Plato alone clearly saw the necessity for the formal cause, for formal causes are, as we have seen, the same as Plato's Ideas. But Plato's philosophy contains only two of the four causes, namely the material and the formal, for Plato made all things out of matter and the Ideas. Since the Ideas have in them [{273}] no principle of motion, Plato's system contains no efficient cause. As for final causes, Plato had indeed the vague idea that everything is for the sake of the Good, but he makes no use of this conception and does not develop it. Final causes were introduced into philosophy by Anaxagoras, whose doctrine of the world forming mind was assumed to explain the design and purpose which the universe exhibits. But as his system developed he forgot about this, and used the Nous merely as a piece of mechanism to explain motion, thus letting it sink into nothing more than an efficient cause.
In the result, Aristotle finds that all four causes have been recognized in greater or lesser degrees by his predecessors, and this, in his opinion, greatly reinforces his own doctrine. But whereas material and efficient causes have been clearly understood, his predecessors had only vaguely foreshadowed and dimly perceived the value of formal and final causes.
The next step in Aristotle's metaphysics is to reduce these four principles to two, which he calls matter and form. This reduction takes place by showing that formal cause, efficient cause, and final cause, all melt into the single conception of form. In the first place, the formal cause and the final cause are the same. For the formal cause is the essence, the concept, the Idea, of the thing. Now the final cause, or the end, is simply the realisation of the Idea of the thing in actuality. What the thing aims at is the definite expression of its form. It thus aims at its form. Its end, final cause, is thus the same as its formal cause. Secondly, the efficient cause is the same as the final cause. For the efficient cause is the cause of becoming. The final cause is the end of [{274}] the becoming, it is what it becomes. And, in Aristotle's opinion, what causes the becoming is just that it aims at the end. The striving of all things is towards the end, and exists because of the end. The end is thus itself the cause of becoming or motion. That is to say, the final cause is the real efficient cause. We may see this better by an example. The end or final cause of the acorn is the oak. And it is the oak which is the cause of the acorn's growth, which consists essentially in a movement by which the acorn is drawn towards its end, the oak. We may see this even more definitely in the case of human productions, because here the striving towards an end is conscious, whereas in nature it is unconscious or instinctive. The efficient cause of the statue is the sculptor. It is he that moves the brass. But what moves the sculptor, and causes him to act upon the brass, is the idea of the completed statue in his mind. The idea of the end, the final cause, is thus the real ultimate cause of the movement. Only, in the case of human production, the idea of the end is actually present in the sculptor's mind as a motive. In nature there is no mind in which the end is conscious of itself, but nevertheless nature moves towards the end, and the end is the cause of the movement. Thus the three causes named all melt into a single notion, which Aristotle calls the form of the thing. And this leaves only the material cause unreduced to any other. So we are left with the single antithesis of matter and form.