The third condition is that in disposing the subject to one of those things to which it is in potentiality, several things should occur, capable of being adjusted in various ways: so as to dispose the subject well or ill to its form or to its operation. Wherefore the simple qualities of the elements which suit the natures of the elements in one single fixed way, are not called dispositions or habits, but "simple qualities": but we call dispositions or habits, such things as health, beauty, and so forth, which imply the adjustment of several things which may vary in their relative adjustability. For this reason the Philosopher says (Metaph. v, text. 24,25) that "habit is a disposition": and disposition is "the order of that which has parts either as to place, or as to potentiality, or as to species," as we have said above (A[1], ad 3). Wherefore, since there are many things for whose natures and operations several things must concur which may vary in their relative adjustability, it follows that habit is necessary.
Reply to Objection 1: By the form the nature of a thing is perfected: yet the subject needs to be disposed in regard to the form by some disposition. But the form itself is further ordained to operation, which is either the end, or the means to the end. And if the form is limited to one fixed operation, no further disposition, besides the form itself, is needed for the operation. But if the form be such that it can operate in diverse ways, as the soul; it needs to be disposed to its operations by means of habits.
Reply to Objection 2: Power sometimes has a relation to many things: and then it needs to be determined by something else. But if a power has not a relation to many things, it does not need a habit to determine it, as we have said. For this reason the natural forces do not perform their operations by means of habits: because they are of themselves determined to one mode of operation.
Reply to Objection 3: The same habit has not a relation to good and evil, as will be made clear further on (Q[54], A[3]): whereas the same power has a relation to good and evil. And, therefore, habits are necessary that the powers be determined to good.