Externalism, transparency, and diagonal propositions

Abstract

Boghossian (Philos Perspect 8:33–50, 1994) argued (a) that externalism is incompatible with a transparency thesis according to which we can know a priori whether any two of our occurrent thoughts have the same or distinct content, and (b) that this transparency thesis is integral to our commonsense conception of rationality, which requires the apriority of our logical abilities. Stalnaker (Our knowledge of the internal world, Oxford University Press, Oxford, 2008) offered a detailed compatibilist response to Boghossian. Boghossian (Philos Stud 155:457–465, 2011) criticized this response, and Stalnaker (Philos Stud 155:467–479, 2011) replied. But the outcome of that important discussion remained unclear, partly because it was unclear how to understand the amended transparency thesis at which Stalnaker was gesturing. My aim in this paper is to settle the matter. I wish (i) to clarify the terms of the debate, also by highlighting the relevant commitments of Stalnaker’s two-dimensionalist account, and (ii) to show that his compatibilist response, which appeals to diagonal propositions, is unsuccessful.