Of course this isn't a fully satisfactory theory of normativity since I don't know what "philosophy" really is (and I'm not even sure it really is a thing).

Empirical approach: first, there is such a thing, because people keep doing it. Second, again, empirically, philosophy, pardon the metaphor, acts like a womb by spawning a natural science from time to time, while not being a science itself. Well, not always natural, sometimes it's logic, or decision theory or something. But mostly. Which would work fine were this goal made explicit: ponder the big questions, chip a small bit of one, make it into a science and let it loose. Unfortunately the practitioners of philosophy (what an oxymoron...) have trouble letting go, and the baby science has to run away and get disowned more often than not.