I answer that, The sensitive powers can be considered in two ways:  first, according as they act from natural instinct:  secondly, according as they act at the command of reason.  According as they act from natural instinct, they are ordained to one thing, even as nature is; but according as they act at the command of reason, they can be ordained to various things.  And thus there can be habits in them, by which they are well or ill disposed in regard to something.

Reply Obj. 1:  The powers of the nutritive part have not an inborn aptitude to obey the command of reason, and therefore there are no habits in them.  But the sensitive powers have an inborn aptitude to obey the command of reason; and therefore habits can be in them:  for in so far as they obey reason, in a certain sense they are said to be rational, as stated in Ethic. i, 13.

Reply Obj. 2:  The sensitive powers of dumb animals do not act at the command of reason; but if they are left to themselves, such animals act from natural instinct:  and so in them there are no habits ordained to operations.  There are in them, however, certain dispositions in relation to nature, as health and beauty.  But whereas by man’s reason brutes are disposed by a sort of custom to do things in this or that way, so in this sense, to a certain extent, we can admit the existence of habits in dumb animals:  wherefore Augustine says (QQ. lxxxiii, qu. 36):  “We find the most untamed beasts, deterred by fear of pain, from that wherein they took the keenest pleasure; and when this has become a custom in them, we say that they are tame and gentle.”  But the habit is incomplete, as to the use of the will, for they have not that power of using or of refraining, which seems to belong to the notion of habit:  and therefore, properly speaking, there can be no habits in them.

Reply Obj. 3:  The sensitive appetite has an inborn aptitude to be moved by the rational appetite, as stated in De Anima iii, text. 57:  but the rational powers of apprehension have an inborn aptitude to receive from the sensitive powers.  And therefore it is more suitable that habits should be in the powers of sensitive appetite than in the powers of sensitive apprehension, since in the powers of sensitive appetite habits do not exist except according as they act at the command of the reason.  And yet even in the interior powers of sensitive apprehension, we may admit of certain habits whereby man has a facility of memory, thought or imagination:  wherefore also the Philosopher says (De Memor. et Remin. ii) that “custom conduces much to a good memory”:  the reason of which is that these powers also are moved to act at the command of the reason.

On the other hand the exterior apprehensive powers, as sight, hearing and the like, are not susceptible of habits, but are ordained to their fixed acts, according to the disposition of their nature, just as the members of the body, for there are no habits in them, but rather in the powers which command their movements. ________________________