 What of option (ii)—denying semantic externalism? Is this really a live prospect for realists? Semantic externalism no longer commands the consensus amongst realists that it did when Putnam formulated his Brains-in-a-Vat argument. David Lewis, a prominent realist, rejected externalism in favour of a sophisticated semantic internalism based on a ‘Two-Dimensional’ analysis of modality proposed by Stalnaker [Lewis 1994]. Frank Jackson [Jackson 2000] contributed to the development of internalist 2D semantics and used it to formulate a version of materialism grounded on conceptual analysis that provides a useful model of a physicalistic realist’s metaphysics.