Why you cannot make people better by telling them what is good

So-called optimists about moral testimony argue, against pessimists, that, ceteris paribus, we ought to accept and act in accordance with trustworthy, pure moral testimony. I argue that even if we grant this, we need to explain why moral testimony cannot make us more virtuous. I offer an explanation that appeals to the fact that we cannot share inferential abilities via testimony. This explanation is compatible with the core commitments of optimism, but it also allows us to see what is right about pessimism.