We define $\neg P=P\rightarrow 0$ in Martin-Lof type theory. So, we have a function $$ (P\to Q)\to [(Q\to 0)\to (P \to 0)] $$ given by function composition. Am I right that there is not reverse function? If so, does that mean contrapositive is not a valid method of argument?
2026-03-09 05:50:06.1773035406
Contrapositive in Type Theory
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