direction. There are exceptions. In the first place we must except

those things which possess a potentiality not in accordance with a

rational principle, as fire possesses the potentiality of giving out

heat, that is, an irrational capacity. Those potentialities which

involve a rational principle are potentialities of more than one

result, that is, of contrary results; those that are irrational are

not always thus constituted. As I have said, fire cannot both heat and

not heat, neither has anything that is always actual any twofold

potentiality. Yet some even of those potentialities which are

irrational admit of opposite results. However, thus much has been said

to emphasize the truth that it is not every potentiality which

admits of opposite results, even where the word is used always in

the same sense.

But in some cases the word is used equivocally. For the term

'possible' is ambiguous, being used in the one case with reference

to facts, to that which is actualized, as when a man is said to find

walking possible because he is actually walking, and generally when

a capacity is predicated because it is actually realized; in the other

case, with reference to a state in which realization is

conditionally practicable, as when a man is said to find walking

possible because under certain conditions he would walk. This last

sort of potentiality belongs only to that which can be in motion,

the former can exist also in the case of that which has not this

power. Both of that which is walking and is actual, and of that

which has the capacity though not necessarily realized, it is true

to say that it is not impossible that it should walk (or, in the other

case, that it should be), but while we cannot predicate this latter

kind of potentiality of that which is necessary in the unqualified

sense of the word, we can predicate the former.

Our conclusion, then, is this: that since the universal is

consequent upon the particular, that which is necessary is also

possible, though not in every sense in which the word may be used.