Within the Summa theologiae, Thomas begins his discussion of human ‘wellfullness’ (Beatitudo) with the first question of the prima secundae. Following the customary orderings of topics within medieval compendiums on theology, Thomas has already within the prima pars, investigated the nature and extent of Sacra Doctrina. This provides a foundation before proceeding on to the secundae pars’s  discussion of human action.
The first question of the prima secundae establishes that human beings act for a telos or end, the ultimate end being God. Thomas believes that, it is with the acquisition of this end that human beings are said to reach a perfected wellfullness or fulfillment. Question 2 of the prima secundae concerns ‘those things, in which man’s happiness consists,’ that is to say, those things needed in order to reach this end (i.e. fulfillment). Articles 1-7 of question 2 establish that humankind’s wellfullness does not consist in natural wealth, honor, human fame/glory, power, any bodily good, pleasure, or even something pertaining to the soul itself. The conclusion thus drawn for Thomas in the course of the articles is that fulfillment cannot be said to consist in the mere acquisition of any created good.
Preliminary Argument 1 (Objection 1) of Ia IIae q.2a.8 contends—based upon a quote from Dionysius (vii)—that, (1) The summit of the lower nature (human) touches the base of the higher nature (the divine) (2) the angels are the base of the higher nature (C) therefore, man’s happiness consists in reaching the status of the angels. The premises of this argument rely heavily on a particular reading of the quotation from Dionysius and are not logically demonstrable proofs. However, in his reply, Thomas demonstrates that even if the premises are taken at face value, the conclusion does not necessarily follow from the premises. Thomas’s reply to Preliminary Argument 1 is that wellfullness will not be reached by simply attaining to the status of the angels. It must seek the ultimate good in order to be perfected. Bearing a kind of likeness to the angelic therefore is merely a way station on the path towards our place in God, not an end within itself.
Preliminary Argument 2 contends—based on Aristotle’s Physics (viii, 2)—that, (1) perfection is the completion of a whole by its parts (2) man is a part of the universe (C) therefore, man’s happiness consists in seeking the good of the universe. Thomas’s reply to Preliminary Argument 2 is that the universe is not an end in itself, but rather is ordained to an end by God. For Thomas, it is a truism that fulfillment can only be found in the last end, because if there exists a further end, then human fulfillment would naturally seek the further end. Thus, the universe cannot be the last end, because God is beyond the universe; to think otherwise, for Thomas, is to seek fulfillment in something which is, in itself, simply vacuous.
Preliminary Argument 3 contends that: (1) a man is made fulfilled (beatus) by the fact that his natural desire comes to rest. (2) But man’s natural desire does not extend to a greater good than he himself is able grasp or to take possession of. (C) Therefore, since man is not capable of a good that exceeds the limits of all of creation (limites totius creaturae); it seems that man can be made fulfilled by some created good. Consequently, man’s beatitude lies in some created good. Thomas’s reply is that created things have good, in so far as they participate in God’s goodness, but this kind of goodness by association, is contracted and limited.
Thomas’s sed contra statement quotes Augustine saying, “As the soul is the life of the body, so God is man’s life of happiness.” In this statement, Augustine is drawing an analogy by saying essentially that, just as the soul is to the body, so God is to the soul. That is, in the same way that the soul vivifies the body, so God vivifies the soul in the form of Grace. Therefore, because humankind’s fulfillment lies within God through the gift of grace, it cannot be said to be located in a created good. The quotation from Psalms 143:15 which closes the sed contra seems rather incidental in light of the former quote.Thomas’s ultimate answer to the question of whether any created good constitutes man’s happiness is a resounding no. This is because, wellfullness for Thomas implies perfection. Perfection is here referred to in the sense of a completion or cessation of human appetite through the acquisition of the will’s proper last end, namely God. For Thomas, the human will necessarily desires its own perfection/fulfillment. This being the case, we are all in a sense hardwired to strive for the universal good. Conversely, if anything were left to be desired beyond God, the human appetite would incline itself to its acquisition. However, God—being the ultimate good—is necessarily the object of all human striving. For Thomas, every creature possesses some measure of goodness, but only in so far as it participates in God’s goodness. Thus, to seek after a created thing as an independent source for one’s own fulfillment is a misguided undertaking, because any individual instance of goodness has to be instantiated by its participation in that which is ‘the good itself.’
 Beatitudo, often translated as ‘happiness’ in English, roughly corresponds to the Greek, εὐδαιμονία (eudemonia), “human flourishing” or “well-being.”
 The first of two divisions of the secundae pars or the ‘Treatise on Ethics,’ which has to do with ethics in general (i.e. the foundations of ethics).
 Beginning with Peter Lombard’s magnum opus, Libri Quattuor Sententiarum, medieval works on theology were oftentimes arranged in four fold divisions treating God, Creation, Christ, and the Sacraments as distinct categories for the purpose of organization. Many scholastic theologians wrote commentaries on the Sentences, including Aquinas’ own early work, Scriptum super libros Sententiarum.
 The first treatise or major division of the Summa theologae.
 Those things pertaining to God, creation, and the government of creatures.
 The second treatise or major division of the Summa theologae.
 Pseudo-Dionysius the Areopagite, De Divinis Nominibus (vii)
 Augustine, De Civitate Dei (xix, 26)
 Thomas assumes, along with Plato and Augustine, that no person can truly will their own harm. Self-destructive behavior is thus the result of a person seeking what seems to them to be their own good through some inordinate or improper means.