What can I know with 100% certainty? I don't actually think you're a twat. Believe it or not, people disagreeing with me isn't inherently something that bothers me. I didn't fall out with corvus because he disagreed with me. If you're a bit of a twat, it's only because it took me so much effort to get you to answer a completely pertinent question - I like clarity and open honesty in these conversations. That really matters to me.
I don't know that I NEED an absolute foundation anyway. I don't agree with the cogito because I NEED to agree with the cogito. I agree with it because I read it and it resonates as truthful. Descartes ran a thought experiment where he doubted everything he could doubt, and then struggled to doubt two final things: he thinks, and he exists. He could doubt physical reality, he could doubt the existence of other minds, he could doubt the existence of gods or dogs or whatever, but if he doubted thought, the wall he hits is that that doubt is a thought...
I don't NEED to be 100% certain of cogito. I would be content being 99.99...% certain of cogito (or less, if there was a reason to be less).
Perhaps it's an intuition, but it's a unique intuition because there are multiple layers of reflexivity in it. It's a self referential intuition, because it's an intuition about the very things that allow you to have intuitions. It's a thought about thought itself. It's existence questioning it's own existence. It gives it a very different flavour from most other institutions. It feels that way to me.