What is something you know, you know for absolutely certain?

[sjel]

I think therefore I am.

Eckhart Tolle has this to say about that quote:

“The philosopher Descartes believed he had found the most fundamental truth when he made his famous statement: "I think, therefore I am." He had, in fact, given expression to the most basic error: to equate thinking with Being and identity with thinking. The compulsive thinker, which means almost everyone, lives in a state of apparent separateness, in an insanely complex world of continuous problems and conflict, a world that reflects the ever-increasing fragmentation of the mind.”

However I cannot say I have separated my thoughts from my sense of self. So I don't know what to believe.