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In pairs/final chains/Poundstone: premises must be checked in pairs (all combinations), because contradictions can be in the transition to the next element: E.g. All grass is green - all hay is brown, hay is grass - "in pairs" that means "takes into account all combinations" - the number of necessary verifications doubles with every adventitious element.
Therefore we cannot check our complex beliefs for consistency - (300 elements: 38 million years of computing time).
I W. Poundstone Im Labyrinth des Denkens, Reinbek 1995