A confusing thing about the meanings of one and many  is that the meaning of each seems to depend on the other. The reality behind this is that there is a back and forth process in which each is used to understand the other better. First we understand being, which is something one, although without the specific idea of unity. Then we understand distinction, which implies several things, again without the specific idea of “many.” Then we understand the one by contrast with things that are distinct. Finally we understand the many as a whole composed of ones as parts.

A similar thing happens with the meanings of “desire” and “good”. Thus St. Thomas defines the good in reference to desire:

I answer that, Goodness and being are really the same, and differ only in idea; which is clear from the following argument. The essence of goodness consists in this, that it is in some way desirable. Hence the Philosopher says (Ethic. i): “Goodness is what all desire.” Now it is clear that a thing is desirable only in so far as it is perfect; for all desire their own perfection. But everything is perfect so far as it is actual. Therefore it is clear that a thing is perfect so far as it exists; for it is existence that makes all things actual, as is clear from the foregoing (3, 4; 4, 1). Hence it is clear that goodness and being are the same really. But goodness presents the aspect of desirableness, which being does not present.

But he also seems to define desire in relation to the good:

I answer that, We must needs assert that in God there is love: because love is the first movement of the will and of every appetitive faculty. For since the acts of the will and of every appetitive faculty tend towards good and evil, as to their proper objects: and since good is essentially and especially the object of the will and the appetite, whereas evil is only the object secondarily and indirectly, as opposed to good; it follows that the acts of the will and appetite that regard good must naturally be prior to those that regard evil; thus, for instance, joy is prior to sorrow, love to hate: because what exists of itself is always prior to that which exists through another. Again, the more universal is naturally prior to what is less so. Hence the intellect is first directed to universal truth; and in the second place to particular and special truths. Now there are certain acts of the will and appetite that regard good under some special condition, as joy and delight regard good present and possessed; whereas desire and hope regard good not as yet possessed. Love, however, regards good universally, whether possessed or not. Hence love is naturally the first act of the will and appetite; for which reason all the other appetite movements presuppose love, as their root and origin. For nobody desires anything nor rejoices in anything, except as a good that is loved: nor is anything an object of hate except as opposed to the object of love. Similarly, it is clear that sorrow, and other things like to it, must be referred to love as to their first principle. Hence, in whomsoever there is will and appetite, there must also be love: since if the first is wanting, all that follows is also wanting. Now it has been shown that will is in God (19, 1), and hence we must attribute love to Him.

This seems circular. Desire is a tendency towards the good, while the good is something that is desirable.

The correct response is that here too we have a back and forth process where each thing makes the other understood better. The first thing in this order is desire, but for the moment without the specific idea of tendency towards the good. Taken in this way, it expresses a way of feeling, a sensible experience. It does not matter here whether we take desire in particular, or its principle, namely love, or its consequence, namely pleasure or joy, or their opposites, such as hate, aversion or sadness. In any case we wish to consider them in a very subjective way, as a way of feeling.

Taken in this way, we can consider them much like a kind of sensation. People sometimes ask how we know that pain is a property of the one who feels pain, rather than of the object that inflicts pain. It seems perfectly possible to say that “this knife is painful” could be just as much an objective fact about the knife, as the fact that the handle of the knife is brown. Of course, no one actually believes this. But the question is why they do not.

It would be easy to suppose that the experiences themselves, namely of seeing the knife and being cut by it, are self explanatory. Of course being cut by a knife is something that happens to me, and of course the color of the knife is a property of the knife.

I agree with the conclusion, naturally, but I do not agree with the reasoning. I do not think that we know this in virtue of the experiences themselves. I think we learn it, very quickly and without a need for conscious attention, from the contexts in which those experiences happen. As I said in the linked post on truth in the senses, sensations are not descriptions of a thing, and they do not make claims. Pain does not assert, “I do not belong to this painful thing”; it does not say anything. Nor does color assert, “I am a property of this body.” It does not say anything. And if we simply consider the sensations as such, we could not give a reason why pain could not be a property of the painful thing, nor why color could not be a property of ourselves rather than the thing. But the contexts in which we have these sensations teach us that color belongs to the colored object, and pain to ourselves, rather than to the painful thing.

Consider the case of sadness. It is easy enough to see that sadness is a property of ourselves, and not of an objectively sad fact. Part of the reason it is easy to see this is that we can be sad, and we can know that we are sad, without noticing any particular reason for being sad. In other words, it is the context of the experience that shows us that it is a property of ourselves.

Something similar is the case with love and desire. Insofar as they are feelings that can be experienced, they can be experienced without noticing any particular object. Katja Grace talks about this situation:

Sometimes I find myself longing for something, with little idea what it is.

This suggests that perceiving desire and perceiving which thing it is that is desired by the desire are separable mental actions.

In this state, I make guesses as to what I want. Am I thirsty? (I consider drinking some water and see if that feels appealing.) Do I want to have sex? (A brief fantasy informs me that sex would be good, but is not what I crave.) Do I want social comfort? (I open Facebook, maybe that has social comfort I could test with…)

If I do infer the desire in this way, I am still not directly reading it from my own mind. I am making educated guesses and testing them using my mind’s behavior.

In this way, it is possible to feel desire as a mere feeling, without defining it in reference to something good. And this kind of feeling is the origin of the idea of “desire,” but it is not yet sufficient.

We learn from experience that when we have desires, we tend to do things. And we notice that not all desires are the same, and that when we have similar desires, we end up doing similar things. And so from this we get the idea of the good as the end and final cause of our actions. We do similar things when we have similar desires, and what those things have in common is that they result in the same ends, even if they use different means. So the end is “why” and explains the choice of means.

In turn, this understanding of the end allows us to understand desire more precisely, now as an inclination towards the good.