If you ask a student of Aristotle's Metaphysics what, on Aristotle's view, makes it that the word, 'exists', refers primarily to 'substance', you will get I think one of two answers:
(i) everything besides substance that exists, exists in a substance; andBut suppose you asked this student: "What, on Aristotle's view, makes it that the word 'exists' is used without qualification (a(plw=j) as regards substance?" Or suppose one generalized the question and asked: "What makes it generally so, that a word is used of something without qualification?" What would the typical reply be in that case?
(ii) to give a definition of any existing thing, one needs to mention a substance.
I'm not sure that there is a generally recognized answer to this question.
But here's something that I found recently when reading Aquinas, which bears upon this. It is a discussion in S.T. I.5.1, where Aquinas is trying to explain how it is that good things are not the same as existing things, even if, as he thinks, things are good just insofar as they are in act.
His position is that for something to exist simpliciter, is for it to be a substance, but for something for it to be good simpliciter, is for it to be perfect: thus, whatever exists simpliciter is therefore good secundum quid, and whatever is good simpliciter therefore exists secundum quid.
But why is it the case that we apply the word 'exists' simpliciter of substances? Here is his explanation:
...cum ens dicat aliquid proprie esse in actu; actus autem proprie ordinem habeat ad potentiam; secundum hoc simpliciter aliquid dicitur ens, secundum quod primo discernitur ab eo quod est in potentia tantum. Hoc autem est esse substantiale rei uniuscuiusque; unde per suum esse substantiale dicitur unumquodque ens simpliciter. Per actus autem superadditos, dicitur aliquid esse secundum quid, sicut esse album significat esse secundum quid: non enim esse album aufert esse in potentia simpliciter, cum adveniat rei iam praeexistenti in actu.
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.Some questions. What is "potential being (or being in potential) simpliciter"? Why does "being" properly signify something that actually is, rather than substance? (If the latter, then there would be no argument.) Does this explanation do any more work than the two explanations above, viz. "being" refers simply to substance, because substance is not the existence of some other existing thing (its being of something would require a qualification in speaking of its existence)? Also, must one find something more basic (as "act" is more basic than "being") and something correlated with that more basic thing (as "potentiality" is correlated with "act") to explain the use of a word simpliciter?