Representation and rationality

David Lewis (1974, 1994/1999) proposed to reduce the facts about mental representation to facts about sensory evidence, dispositions to act, and rationality. Recently, Robert Williams (2020) and Adam Pautz (2021) have taken up and developed Lewis's project in sophisticated and novel ways. In this paper, we aim to present, clarify, and ultimately object to the core thesis that they all build their own views around. The different sophisticated developments and defenses notwithstanding, we think the core thesis is vulnerable. We pose a dilemma by considering the two sides of a current epistemological controversy over the relation between evidence and rational belief: permissivism vs. uniqueness. As we argue, the prospects for the Lewisian project look dim when either supposition is clearly made.