Because idiots are going to idiot, I may as well explain this.

Some things are doubtful to various degrees, and some things are not.

Everyone knows that the question to ask the smug high-school kind who says “Nothing is certain!” is “Are you certain of that?”

Everyone also knows that they are certain of a large number of things indeed.

Descartes was the first to EMPHASIZE this at the beginning of modern philosophy, with his famous COGITO, SUM, “I think; I am!”

Everyone is CERTAIN of his own existence.

It is obviously INCOHERENT not to be certain of one’s own existence, because the very act of attempting to entertain doubts of your existence is YOUR ACT of which your are at the same time IMMEDIATELY AWARE—which proves your existence.

So you know with certainty that you exist, if you are capable of even entertaining the question.

But that is not all.

You are ALSO certain of everything you experience.

If you think you are (phenomenally) in pain, you are in pain. You cannot be mistaken about this.