A question for my readers: Aquinas quotes numerous times Averroes definition of a habit as "that by which one acts when one wills", and seemingly relies on this definition when he argues, for instance, that the habits animals acquire are not habits in the full sense, since "they do not have the power to use or not use them, as seems to belong to the account of a habit." I do not know of any passage where he justifies this part of the definition of habit as helping to make a clear and systematic treatment of the principles of human action.
I have a number of difficulties with this restriction of the term habit: (1) It does not apply to "habits" of being, such as health or beauty; (2) it does not seem to belong in a meaningful sense to natural habits such as synderesis; (3) in the case of men, the freedom to act or not to act doesn't seem any more applicable to a habit than to other, less stable inclinations or disposition to action–if anything, it seems less applicable.
(Update: number 1 above could be explained by the fact that the definition "that by which one acts when one wills" is meant only to define habits of action, not habits of being; still, the fact that definition does not apply to "habits" of being is at least an indication that it is not included in the meaning of the term habit, or the Latin "habitus" — which is related to "habere" and "se habere").
My question is, then, has St. Thomas simply adopted a linguistic usage of the term "habitus" from Averroes' Commentary on Aristotle, or is there some real justification for the insertion of this phrase "that by which one acts one when wills" into the definition of a habit?