The aim of the present article is to accomplish two things. The first is to show that given some further plausible assumptions, existing challenges to the indispensability of knowledge in causal explanation of action fail. The second is to elaborate an overlooked and distinct argument in favor of the causal efficacy of knowledge. In short, even if knowledge were dispensable in causal explanation of action, it is still indispensable in causal explanation of other mental attitudes and, in particular, some reactive (...) attitudes and factive emotions. Taking into account this sort of causal efficacy in determining which mental states are genuine mental states opens up new perspectives for defending the view that knowledge is the most general factive and genuine mental state. (shrink)