THREE:As a result of the foregoing criticism, Aristotle distinguishes four different causes or principles by which all things are determined to be what they areMatter, Form, Agent, and Purpose.226 If, for example, we take a saw, the matter is steel; the form, a toothed blade; the agent or cause of its assuming that shape, a smith; the purpose, to divide wood or stone. When we have enumerated these four principles, we have told everything that can be known about a saw. But Aristotle could not keep the last three separate; he gradually extended the definition of form until it absorbed, or became identified with, agent and purpose.227 It was what we should call the idea of function that facilitated the transition. If the very essence or nature of a saw implies use, activity, movement, how can we define it without telling its purpose? The toothed blade is only intelligible as a cutting, dividing instrument. Again, how came the saw into being? What shaped the steel into that particular form? We have said that it was the smith. But surely that is too vague. The smith is a man, and may be able to exercise other trades as well. Suppose him to be a musician, did he make the saw in that capacity? No; and here comes in a distinction which plays an immense part in Aristotles metaphysics, whence it has passed into our every-day speech. He does not make the saw qua musician but qua smith. He can, however, in the exercise of his trade as smith make many other toolsknives, axes, and so forth. Nevertheless, had he only learned to make saws it would be enough. Therefore, he does not make335 the saw qua axe-maker, he makes it qua saw-maker. Nor, again, does he make it with his whole mind and body, but only with just those thoughts and movements required to give the steel that particular shape. Now, what are these thoughts but the idea of a saw present in his mind and passing through his eyes and hands, till it fixes itself on the steel? The immaterial form of a saw creates the real saw which we use. Let us apply the preceding analogies to a natural object; for example, a man. What is the Form, the definition of a man? Not a being possessing a certain outward shape, for then a marble statue would be a man, which it is not; nor yet a certain assemblage of organs, for then a corpse would be a man, which, according to Aristotle, criticising Democritus, it is not; but a living, feeling, and reasoning being, the end of whose existence is to fulfil all the functions involved in this definition. So, also, the creative cause of a man is another man, who directly impresses the human form on the material supplied by the female organism. In the same way, every definite individual aggregate becomes what it is through the agency of another individual representing the same type in its perfect manifestation.228The first principle to be pointed out in regard to belts, to distinguish them from shafts as a means of transmitting power, is that power is communicated by means of tensile instead of torsional strain, the power during transmission being represented in the difference of tension between the driving and the slack side of belts. In the case of shafts, their length, or the distance to which they may be extended in transmitting power, is limited by torsional resistance; and as belts are not liable to this condition, we may conclude that unless there are other difficulties to be contended with, belts are more suitable than shafts for transmitting power throughout long distances. Belts suffer resistance from the air and from friction in the bearings of supporting pulleys, which are necessary in long horizontal belts; with these exceptions they are capable of moving at a very high rate of speed, and transmitting power without appreciable loss.
