Ideas of Alonzo Church, by Theme

[American, 1903 - 1995, Professor at Princeton University.]

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5. Theory of Logic / K. Features of Logics / 7. Decidability
Validity is provable, but invalidity isn't, because the model is infinite
     Full Idea: Church showed that logic has a proof procedure, but no decision procedure. If an argument is invalid, there is a model with true premises and false conclusion, but the model will typically be infinite, so there is no way to display it concretely.
     From: report of Alonzo Church (A Note on the entscheidungsproblem [1936]) by Vann McGee - Logical Consequence 5