Sure, it sounds pretty reasonable. I mean, it's an elementary facet of logic, and there's no way it's wrong. But, are you really, 100% certain that there is no possible configuration of your brain which would result in you holding that A implies not A, while feeling the exact same subjective feeling of certainty (along with being able to offer logical proofs, such that you feel like it is a trivial truth of logic)? Remember that our brains are not perfect logical computers; they can make mistakes. Trivially, there is some probability of your brain entering into any given state for no good reason at all due to quantum effects. Ridiculously unlikely, but not literally 0. Unless you believe with absolute certainty that it is impossible to have the subjective experience of believing that A implies not A in the same way you currently believe that A implies A, then you can't say that you are literally 100% certain. You will feel 100% certain, but this is a very different thing than actually literally possessing 100% certainty. Are you certain, 100%, that you're not brain damaged and wildly misinterpreting the entire field of logic? When you posit certainty, there can be literally no way that you could ever be wrong. Literally none. That's an insanely hard thing to prove, and subjective experience cannot possibly get you there. You can't be certain about what experiences are possible, and that puts some amount of uncertainty into literally everything else.