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Thomas B. Passin wrote, > I do not think that ordinary business applications today come close > to using full FOL. Mostly things run on ordinary assertions (this > would cover relational databases), and sometimes on rulesets. > Ordinary markup languages are EC - Existential Conjunctive - only one > sector of FOL - and they are almost always enough for these kinds of > uses. I disagree. Actually, I'll go further. Even full first-order logic is too restrictive to express many trivial inferences straightforwardly. For example, There's a 5% surcharge on primary coloured widgets x is a red widget red is a primary colour therefore, There's a 5% surcharge on x This is easy to express in second-order logic, or in first-order logic plus set theory (the "plus set theory" bit means that you can forget about decidability). Not so easy in plain first-order logic. Expressive power, decidablity ... pick one. Cheers, Miles
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