in those things which are potent irrationally, as fire is calorific, and has irrational power; rational powers then are those of many things, and of the contraries; but not all irrational powers, for, as we have said, fire cannot heat, and not heat, nor such other things as always energize. Yet even some irrational powers can at the same time receive opposites; but this has been stated by us, because not every power is susceptible of contraries, not even such as are predicated, according to the same species. Moreover, some powers are equivocal, for the possible is not predicated, simply; but one thing is (called so), because it is true, as being in an energy, as it is possible for a man to walk, because he walks, and in short, a thing is possible to be, because that is already in energy which is said to be possible; on the other hand, another thing (is said to be possible), because it may be in energy; as it is possible to walk, because a man may walk. Now this power exists in movable natures only, but that in immovable; but with respect to both, it is true to say, that it is not impossible to walk or to be, and that a man is now walking and energizing, and has the power to walk, hence it is not true to predicate that which is thus possible, in respect of necessary matter, simply, but the other is true. Wherefore since the universal follows the particular, to be able to be, but not all ability, follows that which is of necessity, and indeed the necessary and the non-necessary may perhaps be
