I'm pretty sure water expands when it freezes, but I can't recall how I know that. Most of our beliefs are like this: we can't remember their justification. But surely it is irrational have a head full of beliefs you cannot justify? I summarise a radical reply from Rene Descartes (1596 - 1650) and a conservative one from Gilbert Harman (b. 1938). Both are weird.

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