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Aristotle, Physics. Book II, Chapter 09

With respect to necessity, however, whether does it subsist from hypothesis, or simply? For now they fancy that a subsistence from necessity is in generation; just as if someone should think that an edifice was raised from necessity, because heavy things are naturally carried downward, and light things upward: and that on this account the stones and foundation of the building are beneath, but the earth above, on account of its levity, and the wood especially in the highest place, because it is most light. At the same time, however, the building is not made without these, and yet not through these; except as through matter; but it is made for the sake of concealing and preserving certain things. The like also takes place in everything else in which there is a subsistence for the sake of something. For they are not, indeed, without these, which have a necessary nature, and yet they are not through or on account of these, except as matter; but they are for the sake of something. Thus, for instance, why is the saw such an instrument as it is? That it may be this thing, and for the sake of this thing. But this subsistence for the sake of something could not be effected, unless it were made of iron. It is necessary, therefore, that it should be made of iron, if it is to be a saw, and the work of it is to have a subsistence. Hence the necessary is from hypothesis, and not as the end: for necessity is in matter; but a subsistence for the sake of something, in reason. But after a certain manner the necessary is similarly in the mathematical disciplines, and in things which are produced according to nature. For since this particular thing is rectilinear, (i.e., a triangle) it is necessary that a triangle should have angles equal to two right; but it does not follow that because it has angles equal to two right it is a triangle, though without the possession of this equality of angles, it is not a triangle. The contrary, however, takes place in things which are produced for the sake of something; for if the end will be, or is, that also which antecedents will be, or is. But if not, as there, when the conclusion is not, the principle will not be, so here likewise the end, and a subsistence for the sake of something, will not be: for this is a principle, not of action, but of reasoning. They are, however, it is the principle of reasoning; For they are not actions. Hence if the house will be, it is necessary that these things should be made, or subsist, or be; or, in short, that matter itself should subsist for the sake of something; as, for instance, stones and tiles, if the house is to be. Yet the end is not on account of these things, except as matter. And, in short, unless these exist, neither the house, nor the saw will have a subsistence; not the former, unless there are stones, nor the latter, unless there is iron. For neither in the above mentioned instance are there principles, unless a triangle has angles equal to two right. It is evident, therefore, that the necessity which is in natural things, is that which is denominated as matter, and the motions of this. And both causes, indeed, are to be considered by the natural philosopher; but in a greater degree that for the sake of which a thing subsists: for this is the cause of matter, but matter is not the cause of the end. The end also, that for the sake of which, and the principle, are assumed from definition and reason. Thus in things artificial, since a house is a thing of a certain description, it is requisite that these particulars should be produced and subsist from necessity; and since health is this particular thing, it is necessary that these things should be generated and exist. Thus too, if man is this particular thing, these particulars must exist; and if the latter is, the former also must have a subsistence. Perhaps also there is the necessary in definition. For the work of the saw, by him who defines it, will be said to be a certain division; but this will not be, unless it has teeth of a certain description; And it will not have these unless it be made of iron: for in definition there are certain parts, which are as it were the matter of the definition.

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