Objection 1. It seems that goodness is prior in idea to being. For names are arranged according to the arrangement of the things signified by the names. But Dionysius (Div. Nom. iii) assigned the first place, amongst the other names of God, to His goodness rather than to His being. Therefore in idea goodness is prior to being.
Objection 2. Further, that which is the more extensive is prior in idea. But goodness is more extensive than being, because, as Dionysius notes (Div. Nom. v), "goodness extends to things both existing and non-existing; whereas existence extends to existing things alone." Therefore goodness is in idea prior to being.
Objection 3. Further, what is the more universal is prior in idea. But goodness seems to be more universal than being, since goodness has the aspect of desirable; whereas to some non-existence is desirable; for it is said of Judas: "It were better for him, if that man had not been born" (Matthew 26:24). Therefore in idea goodness is prior to being.
Objection 4. Further, not only is existence desirable, but life, knowledge, and many other things besides. Thus it seems that existence is a particular appetible, and goodness a universal appetible. Therefore, absolutely, goodness is prior in idea to being.
On the contrary, It is said by Aristotle (De Causis) that "the first of created things is being."
I answer that, In idea being is prior to goodness. For the meaning signified by the name of a thing is that which the mind conceives of the thing and intends by the word that stands for it. Therefore, that is prior in idea, which is first conceived by the intellect. Now the first thing conceived by the intellect is being; because everything is knowable only inasmuch as it is in actuality. Hence, being is the proper object of the intellect, and is primarily intelligible; as sound is that which is primarily audible. Therefore in idea being is prior to goodness.
Reply to Objection 1. Dionysius discusses the Divine Names (Div. Nom. i, iii) as implying some causal relation in God; for we name God, as he says, from creatures, as a cause from its effects. But goodness, since it has the aspect of desirable, implies the idea of a final cause, the causality of which is first among causes, since an agent does not act except for some end; and by an agent matter is moved to its form. Hence the end is called the cause of causes. Thus goodness, as a cause, is prior to being, as is the end to the form. Therefore among the names signifying the divine causality, goodness precedes being. Again, according to the Platonists, who, through not distinguishing primary matter from privation, said that matter was non-being, goodness is more extensively participated than being; for primary matter participates in goodness as tending to it, for all seek their like; but it does not participate in being, since it is presumed to be non-being. Therefore Dionysius says that "goodness extends to non-existence" (Div. Nom. v).
Reply to Objection 2. The same solution is applied to this objection. Or it may be said that goodness extends to existing and non-existing things, not so far as it can be predicated of them, but so far as it can cause them -- if, indeed, by non-existence we understand not simply those things which do not exist, but those which are potential, and not actual. For goodness has the aspect of the end, in which not only actual things find their completion, but also towards which tend even those things which are not actual, but merely potential. Now being implies the habitude of a formal cause only, either inherent or exemplar; and its causality does not extend save to those things which are actual.
Reply to Objection 3. Non-being is desirable, not of itself, but only relatively--i.e. inasmuch as the removal of an evil, which can only be removed by non-being, is desirable. Now the removal of an evil cannot be desirable, except so far as this evil deprives a thing of some being. Therefore being is desirable of itself; and non-being only relatively, inasmuch as one seeks some mode of being of which one cannot bear to be deprived; thus even non-being can be spoken of as relatively good.
Reply to Objection 4. Life, wisdom, and the like, are desirable only so far as they are actual. Hence, in each one of them some sort of being is desired. And thus nothing can be desired except being; and consequently nothing is good except being.
Source: Summa Theologica (New Advent)