In this paper I use methods from cognitive psychology to address a long-standing debate in semantics about the nature of evidential meaning. According to the traditional view, evidentiality grammaticalizes the source of knowledge. Competing modal analyses argue that evidentiality encodes the speaker's subjective evaluation of information in terms of its believability. In an experimental study I hold knowledge source constant and vary the type of logical argument, deductive versus abductive. I find that evidential choices are influenced by both the type of the argument and the perceived subjective strength of the argument. These results are predicted by modal analyses but not by the traditional view. Implications for research on evidentiality in cognitive psychology are discussed.