At Sophist 247e, Plato puts the following into the mouth of the Eleatic Stranger:
I suggest that anything has real being that is so constituted as to possess any sort of power either to affect anything else or to be affected, in however small a degree, by the most insignificant agent, though it be only once. I am proposing as a mark to distinguish real things that they are nothing but power. (Cornford tr.)
The gist of the passage is that what makes a thing real or existent is its (active) power to affect other things or its (passive) power to be affected by them. In sum,
D. For any x, x exists =df x is causally active or passive.
Thus everything causally active/passive exists, and only the causally active/passive exists. The definition rules out of existence all 'causally inert' items such as propositions as Frege construes them, namely, as the senses of context-free indicative sentences. And of course it rules out sets of Fregean propositions. But what about the mathematical (as opposed to commonsense) set of the books on my desks? Each of the books is existent or real by (D) and so is the object resulting from the bundling of the books together; but the set of these books is arguably abstract and thus causally inert. So if (D) is true, we cannot admit mathematical sets into our ontology. For such items do not enter into causal relations. Fregean propositions and mathematical sets are therefore putative counterexamples to (D). If these counterexamples are genuine then (D) fails extensionally: the extension of the existent is wider than the extension of the causally active/passive.
But what interests me at the moment is not the extensional correctness of (D) but a deeper question. Even if we assume that (D) is extensionally correct, i.e., that all and only existents are causally active/passive, does (D) tell us what it is for an item to exist? When we say of a thing that it exists, what are we saying about it? That it is causally active/passive? My answer is in the negative -- even if we assume that all and only existents are causally active/passive.
My reason is quite simple. For an item to be capable of acting or being acted upon it must 'be there' or exist! 'Before' it can be a doer or a done-to it must exist. (The 'before' is to be taken logically not temporally.) The nonexistent cannot act or be acted upon. There is no danger that winged horses will collide with airplanes. The reason is not that winged horses are abstract or causally inert objects; the reason is that they do not exist. Winged horses, if there were any, would belong to the category of the causally active/passive. But they don't exist -- which is the reason why they cannot act or be acted upon. They are not abstract items but nonexistent concrete items. Existence, therefore, is a necessary condition of an item's being a causal agent or patient. It follows that existence cannot be explicated in terms of power as per the Eleatic Stranger's suggestion. Existence is too fundamental to be explicated in terms of power -- or anything else.
If you are having trouble seeing the point consider the winged horse Pegasus and his singleton {Pegasus}. Both of these items are nonexistent. One is concrete (causally active/passive) while the other is abstract. But neither can enter into causal relations. To say that Pegasus is concrete is to say that Pegasus, were he to exist, would belong among the causally active/passive. What prevents him from being such is his nonexistence. His existence, therefore, cannot be explicated in terms of causal activity/passivity.
There is a tendency to conflate two different questions about existence. One question about existence concerns what exists. Answers to this question can be supplied in the form of definitions like (D) above. But there is a deeper question about existence, namely, the question as what it is for an existing thing to exist. What I have just argued is that this second question cannot be answered with any definition like (D). For even if you find a definition that is extensionally correct and immune to counterexamples, you will at the very most have specified the necessary and sufficient conditions for a thing's being among existing things. You will have not thereby have put your finger on what it is for an existing item to exist.
Suppose you say that,
For any x, x exists =df x has properties.
This proposal has an excellent chance of being extensionally correct: necessarily, everything that exists has properties, and everything that has properties exists. But the proposal does not get at the existence of an existing thing precisely because it presupposes the existence of existing things. This is because all such definitions are really circular inasmuch as they have the form:
For any x, x exists =df x is ____ and x exists.
Existence itself eludes definitional grasp. Even if the existent can be defined, the Existence of the existent cannot be defined. For more on this fascinating topic, see my A Paradigm Theory of Existence (Kluwer 2002), pp. 2-8.