I dont claim this is the correct measure of hardness. Why near you may ask the mind of a mathematician provides that impossible has a number therefore nothing is impossible. As a matter of fact Smullyans colleague an MIT logic professor named George Boolos called it the Hardest Logic Puzzle Everand we can see why.
And theres the rub as Hamlet did say and in the same speech.
Riemann took forward the Eulers zeta function to all complex numbers barring s 1. I dont claim this is the correct measure of hardness. A problem is class P if there exists an efficient algorithm to solve it and it is not-P if there isnt. And theres the rub.