Objection 1. It seems that goodness differs really from being. For Boethius says (De Hebdom.): "I perceive that in nature the fact that things are good is one thing: that they are is another." Therefore goodness and being really differ.
Objection 2. Further, nothing can be its own form. "But that is called good which has the form of being", according to the commentary on De Causis. Therefore goodness differs really from being.
Objection 3. Further, goodness can be more or less. But being cannot be more or less. Therefore goodness differs really from being.
On the contrary, Augustine says (De Doctr. Christ. i, 42) that, "inasmuch as we exist we are good."
I answer that, Goodness and being are really the same, and differ only in idea; which is clear from the following argument. The essence of goodness consists in this, that it is in some way desirable. Hence the Philosopher says (Ethic. i): "Goodness is what all desire." Now it is clear that a thing is desirable only in so far as it is perfect; for all desire their own perfection. But everything is perfect so far as it is actual. Therefore it is clear that a thing is perfect so far as it exists; for it is existence that makes all things actual, as is clear from the foregoing (3, 4; 4, 1). Hence it is clear that goodness and being are the same really. But goodness presents the aspect of desirableness, which being does not present.
Reply to Objection 1. Although goodness and being are the same really, nevertheless since they differ in thought, they are not predicated of a thing absolutely in the same way. Since being properly signifies that something actually is, and actuality properly correlates to potentiality; a thing is, in consequence, said simply to have being, accordingly as it is primarily distinguished from that which is only in potentiality; and this is precisely each thing's substantial being. Hence by its substantial being, everything is said to have being simply; but by any further actuality it is said to have being relatively. Thus to be white implies relative being, for to be white does not take a thing out of simply potential being; because only a thing that actually has being can receive this mode of being. But goodness signifies perfection which is desirable; and consequently of ultimate perfection. Hence that which has ultimate perfection is said to be simply good; but that which has not the ultimate perfection it ought to have (although, in so far as it is at all actual, it has some perfection), is not said to be perfect simply nor good simply, but only relatively. In this way, therefore, viewed in its primal (i.e. substantial) being a thing is said to be simply, and to be good relatively (i.e. in so far as it has being) but viewed in its complete actuality, a thing is said to be relatively, and to be good simply. Hence the saying of Boethius (De Hebrom.), "I perceive that in nature the fact that things are good is one thing; that they are is another," is to be referred to a thing's goodness simply, and having being simply. Because, regarded in its primal actuality, a thing simply exists; and regarded in its complete actuality, it is good simply--in such sort that even in its primal actuality, it is in some sort good, and even in its complete actuality, it in some sort has being.
Reply to Objection 2. Goodness is a form so far as absolute goodness signifies complete actuality.
Reply to Objection 3. Again, goodness is spoken of as more or less according to a thing's superadded actuality, for example, as to knowledge or virtue.
Source: Summa Theologica (New Advent)