Thomas Aquinas

“ The former pleasure seems to belong to soul alone: whereas the latter belongs to both soul and body: because the sense is a power seated in a bodily organ: wherefore sensible good is the good of the whole composite. Now concupiscence seems to be the craving for this latter pleasure, since it belongs to the united soul and body, as is implied by the Latin word "concupiscentia." Therefore, properly speaking, concupiscence is in the sensitive appetite, and in the concupiscible faculty, which takes its name from it. ”

“ It belongs to each power of the soul to seek its proper good by the natural appetite, which does not arise from apprehension. But the craving for good, by the animal appetite, which arises from apprehension, belongs to the appetitive power alone. And to crave a thing under the aspect of something delightful to the senses, wherein concupiscence properly consists, belongs to the concupiscible power. ”

“ Because concupiscence of the end is always infinite: since the end is desired for its own sake, e.g. health: and thus greater health is more desired, and so on to infinity; just as, if a white thing of itself dilates the sight, that which is more white dilates yet more. On the other hand, concupiscence of the means is not infinite, because the concupiscence of the means is in suitable proportion to the end. Consequently those who place their end in riches have an infinite concupiscence of riches ”

“ Natural concupiscence cannot be actually infinite: because it is of that which nature requires; and nature ever tends to something finite and fixed. Hence man never desires infinite meat, or infinite drink. But just as in nature there is potential successive infinity, so can this kind of concupiscence be infinite successively; so that, for instance, after getting food, a man may desire food yet again ”

“ 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." ”

“ The reason is possessed of infinite power, in a certain sense, in so far as it can consider a thing infinitely, as appears in the addition of numbers and lines. Consequently, the infinite, taken in a certain way, is proportionate to reason. In fact the universal which the reason apprehends, is infinite in a sense, inasmuch as it contains potentially an infinite number of singulars.
Reply to Objection 3: In order that a man be delighted, there is no need for him to realize all that he desires: for he delights in the realization of each object of his concupiscence. ”

“ 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. ”

“ The diversity, derived from the nature of the active object, causes a material difference of passions: while the difference in regard to its active power causes a formal diversity of passions, in respect of which the passions differ specifically. ”

“ 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 ”

“ 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 ”