This doesn’t mean that philosophy robotically challenges every statement with “why” or that it sees every claim to truth as dubious and open to debunking. Both attitudes are immature – the first being childish and the second being juvenile.
Philosophy challenges everything as a side effect of philosophers wanting to see things for themselves. They are less willing than others to take a large, consensus-based stock of truth for granted and work from it. This is a good that comes at a cost. Philosophical progress can happen, but it is nowhere near as dramatic as the progress one finds in visual art, engineering, experimental science, etc. Even mathematics seems to have achieved progress by taking stocks of truth for granted – one does not get the sense that all that many people could know the supporting proofs for Wiles’s proof of Fermat’s last theorem.
Hypothesis: Philosophy historically separated itself from science around the time when it became clear that, in order to make any progress in a field of knowledge, we would have to take certain stores of knowledge for granted and work from them in order to get anywhere. Somewhere in the first half of the 19th century it became clear that the idea of the liberally educated man, who could see a whole system of knowledge all the way to the bottom, was becoming more and more of a practical impossibility. Before that, we only had “philosophy”, that is, a system that could go all the way from first principles of knowledge to the last things known. But at some point knowledge advanced to the point where this accomplishment, while always the province of a few, became impossible even for those few to accomplish. At this point “philosophy” had to become not a knowing of the whole of things, but only seeing the foundations of things for oneself. Others chose to take their foundations more or less for granted (leaving aside moments of scientific revolution), and to base their knowledge on a set of results achieved collectively and which each person would not even try to see for himself.
And so the old ideal of a liberally educated person became a practical impossibility, and we had to divide the whole of knowledge into philosophy and particular arts and sciences. We haven’t figured out how to re-build the old vision. The clearest proof of this is the utter bewilderment of all liberal-arts curricula in the face of math and science. It is only mildly cynical to see the modern division of “humanities” and “sciences” as a division into “things we can teach in a liberal arts curriculum” and “things we know have to be a part of the liberal arts, but which we have no idea how to incorporate”. Briefly, “humanities” is something with a known syllabus and “sciences” is everything else.
This radical division of the whole left philosophers as only able to question the whole – to question everything. It was the only appropriate response in the face of the magnitude of knowledge advancing beyond the point that a single mind could comprehend it. This is at the heart of the Postmodern sense of utter dissolution of things. It’s not that society is collapsing but something much worse. There will be no grand apocalypse to save us from this crisis, and from which we can start over from the beginning. We’ve discovered a problem in the attempt to know the whole, and that what was called philosophy from Aristotle to Aquinas to Descartes to Newton to Christian Wolfe and Kant was never possible, or that it was only possible because of our relative amount of ignorance. Something of the old liberal idea remains, to be sure, but no one is quite sure how to teach very large parts of the curriculum, or even if it is possible to do so.