How do we define what is good? Is it good because I desire it? Or because I say it is good? Or because we vote and most of us say it is good? Thomas Aquinas gives us an answer in the Summa Theologica.
Good is good because it has perfection. Evil is a lack of perfection, as a man is said to be evil because he lacks some perfection, such as a lack of virtue. A man who was completely virtuous would be good, for then he would have the perfection of virtue. Good, then, is defined as being (act), in the sense that a being (thing) is perfected for its purpose. When a being has total perfection, total being, it is perfect and good.
As Thomas states, “Every being, as being, is good. For all being, as being, has actuality and is in some way perfect; since every act implies some sort of perfection; and perfection implies desirability and goodness, as is clear from Article . Hence it follows that every being as such is good.” (I.5.3) In the reply to the objection that some things are evil, he says “No being can be spoken of as evil, formally as being, but only so far as it lacks being. Thus a man is said to be evil, because he lacks some virtue . . .” (I.5.3.R2)
Note that Thomas is not saying that evil is good, but rather saying that the things which have perfected their form have, in that sense, goodness. He explains, “Everything is said to be good so far as it is perfect; for in that way only is it desirable (as shown above Articles ,3). Now a thing is said to be perfect if it lacks nothing according to the mode of its perfection. But since everything is what it is by its form (and since the form presupposes certain things, and from the form certain things necessarily follow), in order for a thing to be perfect and good it must have a form, together with all that precedes and follows upon that form.” (I.5.5)
We also know that all contingent beings (things) need a necessary being as their cause, and that composed beings cannot cause their own existence. Thus goodness, as perfected being (act), requires a necessary being as the first cause of form and being (act). I believe this is included when he states that “form presupposes certain things.” Further, all this does not take away the act of secondary or proximate causes, such as humans.
We therefore cannot have good without a first necessary being, which we call God.
It is much easier to merely say that good is what God makes to be good, but many ask for a more detailed explanation.