How do you know that your memory of your entire life up the point that you're reading THIS sentence wasn't fabricated, and this is the only moment in time that ever has existed and ever will?
Well, I can't know it for absolute sure, because nothing comes as absolute sure (apart from truisms - including elaborate truisms such as mathematical proofs), as you say. But 'know' does have a meaning in common usage which describes something one
can do. This should be no surprise; a word that describes an impossible act is surely bound to fall out of usage, unless the people who supposedly do that act are fictional (and popular) which, in the case of 'knowing', they are not. I'd suggest that 'know' as commonly used means, if unpacked somewhat, something like 'have enough confidence in the truth (i.e. useful predictiveness) of this claim that I'm happy to base my future behaviour on its correctness'. I'm not saying that's exactly what everyone implies with 'know', and certainly not that people necessarily always have conscious access to that way of describing what they mean by 'know'. But I am saying that people use 'know' meaningfully and pragmatically despite the technical unknowability of everything.
Just because nothing is known certainly, that does not mean that one cannot estimate the probabilities of certain explanations and outcomes, given the evidence; and one can choose between competing explanations on the basis of which is best able to predict future outcomes with reasonable outcomes. If our memories of our past are so wholly fictional, as you suggest in your example 'unknowable', our future could presumably also change arbitrarily and whimsically with no regard to causality. Well, that's easily tested. *does a few things; notes their relatively predictable consequences* Well, as far as I'm concerned, physics and psychology have between them better, more predictive theories on the nature of time and memory than TheAppleCore, on this occasion.
(<--- Smilies to indicate very much no offence or antagonism intended here, and certainly no disrespect to you. I'm just meaning to be playfully debatey.
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