Prior Analytics - Book I   

Having made these distinctions we next point out that the expression

'to be possible' is used in two ways. In one it means to happen

generally and fall short of necessity, e.g. man's turning grey or

growing or decaying, or generally what naturally belongs to a thing

(for this has not its necessity unbroken, since man's existence is not

continuous for ever, although if a man does exist, it comes about

either necessarily or generally). In another sense the expression

means the indefinite, which can be both thus and not thus, e.g. an

animal's walking or an earthquake's taking place while it is

walking, or generally what happens by chance: for none of these

inclines by nature in the one way more than in the opposite.

That which is possible in each of its two senses is convertible into

its opposite, not however in the same way: but what is natural is

convertible because it does not necessarily belong (for in this

sense it is possible that a man should not grow grey) and what is

indefinite is convertible because it inclines this way no more than

that. Science and demonstrative syllogism are not concerned with

things which are indefinite, because the middle term is uncertain; but

they are concerned with things that are natural, and as a rule

arguments and inquiries are made about things which are possible in

this sense. Syllogisms indeed can be made about the former, but it

is unusual at any rate to inquire about them.

These matters will be treated more definitely in the sequel; our

business at present is to state the moods and nature of the

syllogism made from possible premisses. The expression 'it is possible

for this to belong to that' may be understood in two senses: 'that'

may mean either that to which 'that' belongs or that to which it may

belong; for the expression 'A is possible of the subject of B' means

that it is possible either of that of which B is stated or of that

of which B may possibly be stated. It makes no difference whether we

say, A is possible of the subject of B, or all B admits of A. It is

clear then that the expression 'A may possibly belong to all B'