Here is the passage I referenced yesterday. Anthony Gottlieb elegantly sums up the nature of the discipline in his introduction to The Dream of Reason - I can see now where I got the title for the two posts on the subject.
The fact is that the history of philosophy is more the history of a sharply inquisitive cast of mind than the history of a sharply defined discipline. The traditional image of it as a sort of meditative science of pure thought, strangely cut off from other subjects, is largely a trick of historical light. The illusion is created by the way we look at the past, and in particular by the way in which knowledge tends to be labelled, chopped up and rel-labelled. Philosophical work is regularly spirited away and adopted by other disciplines. Yesterday's moral philosophy becomes tomorrow's jurisprudence or welfare economics; yesterday's philosophy of mind becomes tomorrow's cognitive science. And the road runs in both directions: new inquiries in other disciplines prompt new questions for the philosophically curious. Tomorrow's economics will be meat for the moral philosophers of the day after. One effect of these shifting boundaries is that philosophical thinking can easily seem to be unusually useless, even for an intellectual enterprise. This is largely because any corner of it that comes generally to be regarded as useful soon ceases to be called philosophy. Hence the illusory appearance that philosophers never make progress.