This article attempts to develop ideas included in Stump and Gallagher's papers on St. Thomas Aquinas' understanding of love. Aquinas based his theory of disinterested love on the difference between 'amor amicitiae' and 'amor concupiscentiae'. It is easy to misunderstand this difference. St. Thomas holds that both 'amor amicitiae' and 'amor concupiscentiae' are two sides of the same coin, or two 'movements' inside one act of love, to use Aquinas' own terms. 'Amor amicitiae' is directed at the 'substance' of a person, to something hidden in the center of a person, which can hardly be grasped and which is the basis for all personal features. This kind of love, Aquinas says, expresses the human desire for union with the beloved. 'Amor concupiscentiae', on the other hand, is directed at something that may be termed 'goodness for the beloved'; it tries to gain values of various kinds for the beloved person. On Aquinas' view, then, each love has two main objects: 'goodness of the person' and 'goodness for the person'. St. Thomas' metaphysics of disinterested love begins with recognizing the coexistence of these two movements within an act of love. According to Aquinas, to love someone with disinterested love means treating that person as an 'other self' - alter ego. This is possible only through 'affective unity' with the beloved. In a condition of 'affective unity' the concept of 'bonum sui' or 'goodness for myself' widens extensively. 'Goodnes for myself' coincides with 'goodness for the beloved', because we have entered a union on the spiritual level. This spiritual union would not be possible without the human capacity for 'extasis' (ecstasy), which in the context of Aquinas' metaphysics of the person denotes a human capacity to exist 'outside oneself' (extra se). These are the fundamental characteristics of Aquinas' theory of love, and they allow him to examine other matters such as the role of intention in the act of love, goodness as a disposition (dispositio), and so on.