Past exam of the logic and set theory course of the University of Oxford 2010 A 2 a ii Solution 2026-10-31
The requested derivation does not follow from the printed axiom set. Use a two-valued matrix interpretation with ordinary material logical implication and designated value , but interpret the unary symbol by the constant function .
Every instance of the first axiom is valid. In the replacement axiom, both and have value , so its whole logical implication is valid as well. Modus ponens rule preserves the designated value. The deduction rule is sound for semantic consequence under this interpretation because logical implication is the ordinary truth-functional logical implication. Induction on derivations, allowing arbitrary sets of assumptions, therefore makes every theorem valid in this matrix.
But the old contraposition axiom has valuewhich is false when and . ConsequentlyThis is a truth-matrix countermodel for a Hilbert axiom set. A nonstandard interpretation of a primitive symbol is legitimate for proving syntactic nonderivability: only preservation by the displayed axioms and rules is required. The source would need a stronger negation axiom or an extra negation principle to support its requested conclusion.