Aristotle, Metaphysics

[1026b] [1] and further besides all these the potential and actual : since the term "being" has various senses, it must first be said of what "is" accidentally, that there can be no speculation about it.This is shown by the fact that no science, whether practical, productive or speculative, concerns itself with it. The man who produces a house does not produce all the attributes which are accidental to the house in its construction; for they are infinite in number. There is no reason why the house so produced should not be agreeable to some, injurious to others, and beneficial to others, and different perhaps from every other existing thing; but the act of building is productive of none of these results.In the same way the geometrician does not study the accidental attributes of his figures, nor whether a triangle is different from a triangle the sum of whose angles is equal to two right angles. And this accords with what we should reasonably expect, because "accident" is only, as it were, a sort of name. Hence in a way Plato¹ was not far wrong in making sophistry deal with what is nonexistent;because the sophists discuss the accident more, perhaps, than any other people—whether "cultured" and "grammatical,"² and "cultured Coriscus" and "Coriscus,"³ are the same or different; and whether everything that is, but has not always been, has come into being, so that if a man who is cultured has become grammatical, [20] he has also, being grammatical, become cultured⁴; and all other such discussions. Indeed it seems that the accidental is something closely akin to the nonexistent.This is clear too from such considerations as the following: of things which are in other senses there is generation and destruction, but of things which are accidentally there is not.⁵ Nevertheless we must state further, so far as it is possible, with regard to the accidental, what its nature is and through what cause it exists. At the same time it will doubtless also appear why there is no science of it.

Since, then, there are among existing things some which are invariable and of necessity (not necessity in the sense of compulsion,⁶ but that by which we mean that it cannot be otherwise⁷), and some which are not necessarily so, nor always, but usually: this is the principle and this the cause of the accidental. For whatever is neither always nor usually so, we call an accident.E.g., if in the dog-days⁸ we have storm and cold, we call it an accident; but not if we have stifling and intense heat, because the latter always or usually comes at this time, but not the former. It is accidental for a man to be white (since this is neither always nor usually so), but it is not accidental for him to be an animal.