Objection 1. It would seem that these four things are not rightly reckoned as to be loved out of charity, to wit: God, our neighbor, our body, and ourselves. For, as Augustine states (Tract. super Joan. lxxxiii), "he that loveth not God, loveth not himself." Hence love of oneself is included in the love of God. Therefore love of oneself is not distinct from the love of God.
Objection 2. Further, a part ought not to be condivided with the whole. But our body is part of ourselves. Therefore it ought not to be condivided with ourselves as a distinct object of love.
Objection 3. Further, just as a man has a body, so has his neighbor. Since then the love with which a man loves his neighbor, is distinct from the love with which a man loves himself, so the love with which a man loves his neighbor's body, ought to be distinct from the love with which he loves his own body. Therefore these four things are not rightly distinguished as objects to be loved out of charity.
On the contrary, Augustine says (De Doctr. Christ. i, 23): "There are four things to be loved; one which is above us," namely God, "another, which is ourselves, a third which is nigh to us," namely our neighbor, "and a fourth which is beneath us," namely our own body.
I answer that, As stated above (23, 1,5), the friendship of charity is based on the fellowship of happiness. Now, in this fellowship, one thing is considered as the principle from which happiness flows, namely God; a second is that which directly partakes of happiness, namely men and angels; a third is a thing to which happiness comes by a kind of overflow, namely the human body.
Now the source from which happiness flows is lovable by reason of its being the cause of happiness: that which is a partaker of happiness, can be an object of love for two reasons, either through being identified with ourselves, or through being associated with us in partaking of happiness, and in this respect, there are two things to be loved out of charity, in as much as man loves both himself and his neighbor.
Reply to Objection 1. The different relations between a lover and the various things loved make a different kind of lovableness. Accordingly, since the relation between the human lover and God is different from his relation to himself, these two are reckoned as distinct objects of love, for the love of the one is the cause of the love of the other, so that the former love being removed the latter is taken away.
Reply to Objection 2. The subject of charity is the rational mind that can be capable of obtaining happiness, to which the body does not reach directly, but only by a kind of overflow. Hence, by his reasonable mind which holds the first place in him, man, out of charity, loves himself in one way, and his own body in another.
Reply to Objection 3. Man loves his neighbor, both as to his soul and as to his body, by reason of a certain fellowship in happiness. Wherefore, on the part of his neighbor, there is only one reason for loving him; and our neighbor's body is not reckoned as a special object of love.
Source: Summa Theologica (New Advent)