[weikel]

You don't know not 100%.

[dyed_in_the_wool]

You don't know not 100%.

When I was majoring in Philosophy, I would administer the 'baseball bat' test whenever someone posited you could not be 100% certain about reality.
Essentially, it works like thus: I start hitting you about the head with a baseball bat until you admit that you are CERTAIN that the bat exists.
It turns out we can be a lot more certain about reality than we think we can if we really put our minds to it.
Realize, I'm no friend of A.J. Ayer, but I've always kept Samuel Johnson's refutation of Bishop Berkeley close at hand. When asked about Berekeley's doubts about the existence of things and what he thought of them, Johnson kicked a stone in reply.
Basically, you might doubt whether you can be certain about the existence of external things, but that doubt does not preclude knowledge. Or reality.

[thinktwice]

You don't know not 100%.

Until proven wrong, every human is certain about ... many, many things.