Skepticism.

[ZenitYerkes]

If I had to put a simile with skepticism, I'd say it's the acid that corrodes one of the main pillars of philosophy: the relation between what we perceive and what's actually in front of us.

You might say, "See this book? I can touch it, see it, hear how it falls to the floor, smell the cellulose of its white sheets and even taste them. I think, this book is really here."

But does that mean the book really exist? Is it really there?

Imagine we were in a Matrix-like reality. That would mean we are not actually feeling by any means that book; it'll all be an illusion from the main control computer. How if everything we're living right now, is not actually what we perceive?

Anyway, being practical, I think it's more useful to take for given that what we perceive is real -you wouldn't be doubting whether that bear running to your place with raging eyes actually exists.