Objection 1. It would seem that habits are not necessary. For by habits we are well or ill disposed in respect of something, as stated above. But a thing is well or ill disposed by its form: for in respect of its form a thing is good, even as it is a being. Therefore there is no necessity for habits.
Objection 2. Further, habit implies relation to an act. But power implies sufficiently a principle of act: for even the natural powers, without any habits, are principles of acts. Therefore there was no necessity for habits.
Objection 3. Further, as power is related to good and evil, so also is habit: and as power does not always act, so neither does habit. Given, therefore, the powers, habits become superfluous.
On the contrary, Habits are perfections (Phys. vii, text. 17). But perfection is of the greatest necessity to a thing: since it is in the nature of an end. Therefore it is necessary that there should be habits.
I answer that, As we have said above (2,3), habit implies a disposition in relation to a thing's nature, and to its operation or end, by reason of which disposition a thing is well or ill disposed thereto. Now for a thing to need to be disposed to something else, three conditions are necessary. The first condition is that which is disposed should be distinct from that to which it is disposed; and so, that it should be related to it as potentiality is to act. Whence, if there is a being whose nature is not composed of potentiality and act, and whose substance is its own operation, which itself is for itself, there we can find no room for habit and disposition, as is clearly the case in God.
The second condition is, that that which is in a state of potentiality in regard to something else, be capable of determination in several ways and to various things. Whence if something be in a state of potentiality in regard to something else, but in regard to that only, there we find no room for disposition and habit: for such a subject from its own nature has the due relation to such an act. Wherefore if a heavenly body be composed of matter and form, since that matter is not in a state of potentiality to another form, as we said in the I, 56, 2, there is no need for disposition or habit in respect of the form, or even in respect of operation, since the nature of the heavenly body is not in a state of potentiality to more than one fixed movement.
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 (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 (54, 3): whereas the same power has a relation to good and evil. And, therefore, habits are necessary that the powers be determined to good.
Source: Summa Theologica (New Advent)