Alfred North Whitehead:

In fact, Whitehead describes any entity as in some sense nothing more and nothing less than the sum of its relations to other entities – its synthesis of and reaction to the world around it. A real thing is just that which forces the rest of the universe to in some way conform to it; that is to say, if theoretically a thing made strictly no difference to any other entity (i.e. it was not related to any other entity), it could not be said to really exist. Relations are not secondary to what a thing is, they are what the thing is.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". How does he describe what makes something real?
A real thing is just that which forces the rest of the universe to in some way conform to it