Aristotle’s ethic was eudaimonistic, which was later developed by Thomas Aquinas. Evil is the negation of good and requires no ontological grounding and it is the case that everyone always acts according to what they believe is good. Thomas’ meta-ethic was that being and goodness are the same in reference but differ only in sense. He follows Aristotle in making the connection between goodness and desirability. “The formula of the good consists in this, that something is desirable, and so the Philosopher [Aristotle] says that the good is what all desire.” Although all things desire goodness, not all things capable of pursuing goodness and pleasure with understanding understand what really is good; it is possible for creatures with intellect and will to desire an apparent good as a real one. Thomas states that something is desirable in two ways, either because it is good or because it appears good. Of these, the first is what is good, for an apparent good does not move by itself but insofar as it has some appearance of good; but the good moves by itself. Desirability and pleasure is an essential aspect of goodness. The perfection of anything is goodness and perfection is attained in actuality, “As regards nature the good of anything is its actuality and perfection.”
Goodness appears in the notion of that which desire culminates: Necessarily, everything which a human being desires, he desires for the sake of the ultimate end… whatever a human being desires he desires under the aspect of the good. And if the good is not desired as the perfect good, which is the ultimate end, it must be desired as instrumental to the perfect good. Every action and every motion is apparently ordered in some way to being, either that being might be conserved in the species or the individual, or that it might be newly acquired. For what being is-is the good. And so everything desires to be. Therefore, every action and every motion is for the sake of the good.
 Thomas Aquinas, Summa Theologica 1a.5.1; Summa Contra Gentiles I37; Aristotle, Nichmachean Ethics I 1, 1094a1-3.
 Ele0nore Stump, Aquinas (New York: Routledge, 2003), 62.
 Thomas, SCG, 1.37
 Ibid. Stump, 62-63.
 Thomas, ST 1aIIae.1.6..
 SCG III.3; Stump, 63.