skepticism

How do you know what you know isn't an illusion?
How do you know what you know is what you know?

Decrates tried to reducing it to an axiomatic “I think, therefore I am”.
Surely if you’re thinking, then you’re existing in the first place to think the thought, right?

Not to Nietzsche, who made the argument that this statement is full of assumptions:
1. It is you who is doing the thinking
2. Thinking at all is taking place
3. There is such thing as an I
4. That we know what thinking is

Furthermore an argument could be made that thinking is a strictly first-person perspective, and as such cannot be used to extend arguments to the real world.