My internal reality I hope tracks external reality.

And what I can be certain about is what my internal reality is.

And if you think I cannot be sure of that, I think I can be certain about I think my internal reality is.

And if you think I cannot be sure of even that, I think I can be certain about I think what I think my internal reality is.

It's perception all the way down, recursively. The reality is result of this taken to infinity.