Fermat lived almost three decades after he proposed his "Last Theorem" and was hardly inactive in that period. Had he even thought he actually had a proof, he w

Fermat lived almost three decades after he proposed his "Last Theorem" and was hardly inactive in that period. Had he even thought he actually had a proof, he would have published it, and we would all know it was wrong. A far more likely scenario is that he had a false proof, discovered he had a false proof at some point after he made his marginal note, and didn't bother to publish the fact he had made a fool of himself in some obscure marginalia. The only mystery surrounding Fermat's understanding of his own "Last Theorem" is precisely which false proof he most likely had.