Objection 1: It would seem that concupiscences are not divided into those which are natural and those which are not. For concupiscence belongs to the animal appetite, as stated above (A. 1, ad 3). But the natural appetite is contrasted with the animal appetite. Therefore no concupiscence is natural.
Obj. 2: Further, material differences makes no difference of species, but only numerical difference; a difference which is outside the purview of science. But if some concupiscences are natural, and some not, they differ only in respect of their objects; which amounts to a material difference, which is one of number only. Therefore concupiscences should not be divided into those that are natural and those that are not.
Obj. 3: Further, reason is contrasted with nature, as stated in Phys. ii, 5. If therefore in man there is a concupiscence which is not natural, it must needs be rational. But this is impossible: because, since concupiscence is a passion, it belongs to the sensitive appetite, and not to the will, which is the rational appetite. Therefore there are no concupiscences which are not natural.
On the contrary, The Philosopher (Ethic. iii, 11 and Rhetor. i, 11) distinguishes natural concupiscences from those that are not natural.
I answer that, As stated above (A. 1), concupiscence is the craving for pleasurable good. Now a thing is pleasurable in two ways. First, because it is suitable to the nature of the animal; for example, food, drink, and the like: and concupiscence of such pleasurable things is said to be natural. Secondly, a thing is pleasurable because it is apprehended as suitable to the animal: as when one apprehends something as good and suitable, and consequently takes pleasure in it: and concupiscence of such pleasurable things is said to be not natural, and is more wont to be called "cupidity."
Accordingly concupiscences of the first kind, or natural concupiscences, are common to men and other animals: because to both is there something suitable and pleasurable according to nature: and in these all men agree; wherefore the Philosopher (Ethic. iii, 11) calls them "common" and "necessary." But concupiscences of the second kind are proper to men, to whom it is proper to devise something as good and suitable, beyond that which nature requires. Hence the Philosopher says (Rhet. i, 11) that the former concupiscences are "irrational," but the latter, "rational." And because different men reason differently, therefore the latter are also called (Ethic. iii, 11) "peculiar and acquired," i.e. in addition to those that are natural.
Reply Obj. 1: The same thing that is the object of the natural appetite, may be the object of the animal appetite, once it is apprehended. And in this way there may be an animal concupiscence of food, drink, and the like, which are objects of the natural appetite.
Reply Obj. 2: The difference between those concupiscences that are natural and those that are not, is not merely a material difference; it is also, in a way, formal, in so far as it arises from a difference in the active object. Now the object of the appetite is the apprehended good. Hence diversity of the active object follows from diversity of apprehension: according as a thing is apprehended as suitable, either by absolute apprehension, whence arise natural concupiscences, which the Philosopher calls "irrational" (Rhet. i, 11); or by apprehension together with deliberation, whence arise those concupiscences that are not natural, and which for this very reason the Philosopher calls "rational" (Rhet. i, 11).
Reply Obj. 3: Man has not only universal reason, pertaining to the intellectual faculty; but also particular reason pertaining to the sensitive faculty, as stated in the First Part (Q. 78, A. 4; Q. 81, A. 3): so that even rational concupiscence may pertain to the sensitive appetite. Moreover the sensitive appetite can be moved by the universal reason also, through the medium of the particular imagination.