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This paper aims to provide criteria for aiding central banks to choose between different monetary policy rules that respond to inflation. Special attention is given to the question of whether monetary policy instruments are predetermined or forward-looking. Based on the example of the New Keynesian Phillips curve, using a non-predetermined instrument when inflation is also non-predetermined indicates an extreme lack of robustness and credibility in price anchoring and stabilization, since central banks do not know the exact parameters of the economy. By contrast, a predetermined instrument ensures that proportional rules anchor non-predetermined inflation in a robust and credible manner. This rule can thus represent a limited version of a Ramsey-optimal policy with minimal credibility and robustness. We recommend modeling optimal policies with minimal central bank credibility instead of proportional rules by viewing both the instruments and targets of monetary policy as non-predetermined.