If you obsess too much about more or less anything, it can be bad for you, but some of the topics that philosophy deals with can be more troublesome for some people. I have in mind especially some forms of radical skepticism. I have met people who've been obsessed with the possibility that they don't really know that there's an external world, or that there are other people. I've had them in my office, clearly distressed. I'd guess that there were already some underlying problems. I'd guess that it wasn't a case of someone who was otherwise fully functioning and then happened on Descartes' Meditations, but the skeptical questions provided something for their background issues to latch onto.

When I'm talking with such folk, I tend to stress two things. The first is philosophical: the fact that something is possible in some rarified sense isn't a reason for thinking it's at all likely. Yes: in some sense of "could," it could be that what's going on in my mind is all there is. But in this sense of "could," there are many, many things that could be true, including the overall possibility that my beliefs are broadly true and the infinitely many more specific variants on that possibility. Most philosophers who think about knowledge take it as given that we know lots and lots of things. The questions they're interested in are about the detailed structure of knowledge, not about whether we actually have any.

But aside from such philosophical commonplaces, when I've been in conversation with people for whom disturbing philosophical possibilities have become intrusive, I also offer a different suggestion: get out of your head. Get some exercise. Listen to music. Have lunch with a friend and talk about something fun. The great Scots philosopher David Hume wrote that he did just this sort of thing. When one is not just being a bystander to one's life, it's harder for philosophy's more outre speculations to hold their grip.