Addendum: true personal certainty is much rarer than one thinks. Strictly speaking I am, I think, only confident that 2 + 2 = 4; very confident, since I know from trustworthy authorities that arithmetic may be proven logically consistent and from experience that categories of twos added together produce fours. Confident enough, even, to bet my life (if such a bet were necessary) that adding two to two in any life-threatening scenariorio would make four. Confidence is a real part of knowledge. But am I certain that 2 + 2 = 4? I do not understand the rules and axiomata of mathematical logic by which such statements are proven logically certain in the strong sense. I could, I suppose, be living in a remarkable delusion which has filtered out all experiences of 2 + 2 failing to equal 4 — which seems highly unlikely, but is that not just what such a deluded person might say? My certainty is “only” functional, which is actually — a species of confidence. Is there anything regarding which I am truly certain?