According to St. Thomas Aquinas: Love is something pertaining to the appetite; since good is the object of both. So love and appetite are words used to describe a spiritual perception that begins in the body and ends in the soul. Appetite is the first instance of love, and thus we move from physical need to spiritual desire. Wherefore love differs according to the difference of appetites. There are different kinds of appetites depending upon the object of desire. For there is an appetite which arises from an apprehension existing, not in the subject of the appetite, but in some other: and this is called the “natural appetite.” This appetite relates to food, drink, and sex. With regard to all three there is always a danger that they might not be pursued according to prudence and right reason. Thus, when one is attracted to another human being in any way there is a kind of natural appetite. “Natural” here refers to the feeling in the body inspired by the other. This is not to say that all ‘natural’ appetites are proper and suitable, such that they can or ought to be pursued. Oftentimes appetites present in the body are misappropriated or misapplied because of some early childhood physical, emotional, or spiritual trauma. Thus we find all sorts of people who have “natural appetites” that are stirred inappropriately or unnaturally. Such come about because the appetites have not been perfected or cultivated appropriately in the process of development from childhood into adulthood. Thus, for example, we find that sexual attraction or appetite might be experienced as an unconscious identification with a part of one’s sexuality that is meant to be affirmed psychologically and intellectually in a platonic fashion. That such has not transpired is the result of a repressed and retarded phase of development that is violently misappropriated and misapplied because of past violent assault and maiming. A perversion of this appetite more often than not results in addiction as it pertains to the respective faculty. Thus we find gluttony, drunkenness, and unchastity as inappropriate and excessive relations to food, drink, or sex.
And there is another appetite arising from an apprehension in the subject of the appetite, but from necessity and not from free-will. Such is, in irrational animals, the “sensitive appetite,” which, however, in man, has a certain share of liberty, in so far as it obeys reason. The sensitive appetite would seem to involve more rather than less mind or intellection with regard to appetites. In this place one finds an ability to discern and desire, or to calculate and choose with regard to the object of desire and the desiring of it. Here one finds a conflation of appetites and thus the need for the mind to choose one over the other, or to choose all but in relation to a higher principle of moderation. So, for example, a man can choose between food, drink, and sex, and can moderate them by placing a time constraint on all and a priority on some. All three, for man, are necessary and yet in need of taming and bridling if a man is to realize his rational potential. The subjection of these natural instincts to the higher end of man’s perfection is essential to the happy life. And thus a man must moderate the needs of his body to the good of his soul. The good of his soul then must compel his reason to rule and govern his baser appetites and instincts. The good of his soul must also move his mind to overcome the imperfections of appetite by surrendering to the means of their correction and discipline. Here appetite must be controlled so that desire for the good overrules the avoidance of pain, the pursuit of pleasure, fleeing fear, and following foolishness. Such is accomplished by submission to temperance, prudence, justice, and fortitude.
Again, there is another appetite following freely from an apprehension in the subject of the appetite. And this is the rational or intellectual appetite, which is called the “will.” The highest kind of appetite then is the desire of the soul to know the origin, source, and cause of its becoming being that is on the way to perfection. Rational or intellectual appetite is the desire to know the underlying causes of all things and then the cause of all cases to be found in God. So this appetite is stirred on by a desire and passion to know and then to love God. To know and to love God is to know oneself in God and God in oneself. It is also to know all created things as coming from and returning to God. So the passion of the mind is a gift to be actualized in the pursuit of knowledge and the gratitude that comes with discovering that knowledge is a gift that is bestowed on the soul that gratefully finds itself as moved by a loving God out of ignorance and into knowledge. The gift of enlightenment is thus discovered to be the blessing that comes from a God who loves to make himself known and present to his inquisitive creature. Here the gifts to be cultivated are faith, hope, and love. In faith a man believes in the power that illuminates him. In hope he longs for deeper and more perfect enlightenment. In love he delights in the gift of an ongoing and ever-expanding sense of the Divine mercy and affiance.
S.T.: I, II, 26, 1. Thomas Aquinas with Commentary