Agere sequitur esse (“action follows being” or “activity follows existence”) is a basic principle of Scholastic metaphysics. The idea is that the way a thing acts or behaves reflects what it is. But suppose that a thing doesn’t truly act or behave at all. Would it not follow, given the principle in question, that it does not truly exist? That would be too quick. After all, a thing might be capable of acting even if it is not in fact doing so. (For example, you are capable of leaving this page and reading some other website instead, even if you do not in fact do so.) That would seem enough to ensure existence. A thing could hardly be said to have a capacity if it didn’t exist. But suppose something lacks even the capacity for acting or behaving. Would it not follow in that case that it does not truly exist?
To affirm that conclusion without qualification would be to endorse what Jaegwon Kim calls Alexander’s Dictum: To be real is to have causal powers. (The dictum is named for Samuel Alexander, who gives expression to the idea in volume 2 of Space, Time, and Deity. Kim has discussed it in many places, e.g. here.)