For most students of philosophy a moment comes when a perspective or prejudice they've held for quite a while gets overturned. Like Hume's problem of induction hits you just right and you think, "oh great, so logic can't even help with my belief in the uniformity of nature?"
In any case, which belief gets changed isn't the point. The point is, from there on you have to face the personal fact that along with the expression I know is the expression I thought I knew (Wittgenstein). What separates philosophers from others is that they live in the perpetual possibility of the turn.
Q: And what of so-called philosophers?
I'm just relieved I didn't limit the set to only those who perpetually pursue the turn.