One answer to the titular question is this: Philosophy is what philosophers do. But this is no help, for until we know who the philosophers are, we don't know what philosophers, as such, do; and we don't know who the philosophers are until we know what philosophy is.
For consider: A given philosopher is many things, not one thing. Take me, for example. I'm a philosopher, a lawyer, an historian, a teacher, a citizen, a husband, a cyclist, and a musician (to mention just a few of my many attributes). Not everything I do is philosophy (thank God!), so it would be invalid (fallacious) to infer "x is philosophy" from "Keith is a philosopher" and "Keith does x." It's true that Keith is a philosopher and it's true that Keith rides a bike, but it's false that riding a bike is philosophy (though, truth be told, I do much philosophy while abike).
As this example shows, what I do may or may not be in my capacity as a philosopher. So if our objective is to answer the question "What is philosophy?," it does no good to answer "Whatever philosophers do." There must be some other way—a noncircular way—to answer the question.
