Whereas both Thomas and Aristotle would agree that “the ultimate end of man, as of any intellectual substance, is called felicity or happiness” there is a profound difference between them regarding in what that happiness consists. The difference between Thomas and Aristotle’s accounts of the human good boils down to the difference between Thomas’ Christianity (grace and faith) and Aristotle’s virtuous paganism (the integrated intellectual life at its humanly best).
For an excellent introductory guide to Aquinas follow this link.
Thomas, as a Christian, says that “it is the happiness and felicity of every intellectual substance to know God” (Thomas Aquinas: Selected Writings, Penguin Books., London 1998, p 267). Crucially, Thomas quotes Scripture citing both Jesus’ teaching and Apostolic teaching to undergird this definition. “Blessed are the clean of heart…” and “the knowledge of God” are both dependent upon grace and revelation. Whereas the grace and revelation proceed from God towards the world through Jesus, the way we enter into the reality of grace and perceive that revelation is by the eyes of faith.
Interestingly, Thomas says that “Aristotle agrees with this judgement” following his explicit statement regarding happiness being to “know God.” According to Thomas, Aristotle’s agreement consists in “speculation bearing on the best speculable object” and yet, for Aristotle, without grace or revelation, this “best speculable object” remained in a personal sense, unknown to him. For all the benefits received from Aristotle’s teaching, and the admiration extended towards him, Dante does not hesitate to place Aristotle in Hell (limbo). For Aristotle, the “best speculable object” was as lofty as one could perceive within the powers of the merely human intellect and thus hell for him, according to Dante, is not punishment for wickedness, but rather knowingly missing out on the highest intellectual vision of God himself. For Aquinas, his intellect was aided by faith, and by faith he was able to perceive the highest good as a personal God.
Flowing from this “the human good” as understood by both Aristotle and Thomas are further differentiated in relation to the virtues. For Aristotle, “The human good turns out to be the soul’s activity that expresses virtue” (Nicomachean Ethics 1098a15). Thomas says “moral virtues are for the sake of preserving the mean in inner passions and external things, but it could not be that the moderating of passions and of external goods is the ultimate end of human life. These passions are ordered to something else…” (Selected Writings p. 278). This ‘something else’ is only known to us by way of grace and revelation, through the eyes of faith. For Thomas, Aristotle’s natural virtues are assumed into the life and light of faith by way of the supernatural virtues of ‘faith,’ ‘hope,’ and ‘love.’ The Good, as they relate to the virtues remain ‘merely human’ for Aristotle, whereas for Thomas, the Good, is the participation in the holiness of God, by grace and faith. Perhaps it could be summed up that Aristotle’s conception of the human good was not good enough, as the natural capacities of the human intellectual soul remain a ceiling, rather than a floor upon which the light of faith could rest in order to proceed further by the grace of God.