Excerpt from an analysis essay of Thomas Aquinas’ Summa Contra Gentiles Chapter 38…
In section 4 of Chapter 38, Aquinas begins by discussing how any good thing, call it A, that exists must be good by participation in some other thing, call it B. In this scenario B must be of a higher order than A because B is the source of A’s goodness; said in another way, any cause must be greater than its effect. Though possible to trace a thing’s goodness back to its participation in another good thing, Aquinas argues that this pattern cannot extend to infinity because infinity is opposed to the end, and good has the nature of an end. Stating “good has the nature of an end” indicates that each good can be traced back to the participation in another, higher good. For this to comply, there must be a first good that doesn’t participate in any other good because there is nothing greater than it. Its source of goodness is its own essence, not participation in any higher good. As it previously argued, God is the only possible entity whose being is the same as its essence. Thus we have come to Aquinas’ conclusion: because good things have the nature of an end, God must be goodness because God’s being is God’s essence.