Is there a deductive argument for semantic externalism? Reply to Yli-Vakkuri

Juhani Yli-Vakkuri (2017) has recently argued that the Twin Earth thought experiments offered in favour of semantic externalism can be replaced by a straightforward deductive argument from premises widely accepted by both internalists and externalists alike. The deductive argument Yli-Vakkuri offers, however, depends on premises which are such that, on standard formulations of internalism, they cannot be satisfied by a single belief simultaneously; it does not therefore, constitute a proof of externalism. The aim of this paper is to explain why.