Abstract. For a reasonable sound and complete proof calculus for firstorder logic consider the problem to decide, given a sentence of firstorder logic and a natural number n, whether has no proof of length n. We show that there is a nondeterministic algorithm accepting this problem which, for fixed , has running time bounded by a polynomial in n if and only if there is an optimal proof system for the set TAUT of tautologies of propositional logic. This equivalence is an instance of a general result linking the complexity of so-called slicewise monotone parameterized problems with the existence of an optimal proof system for TAUT.