 The slow switch thought experiment again features Oscar on Earth—yet this time, we suppose he is unwittingly switched to Twin Earth (e.g., during his sleep by a secret government operative) (Burge 1988, Boghossian 1989). Upon his arrival, he then takes up his usual activities, being none the wiser. Now the externalist’s intuition is that although Oscar’s use of‘water’ on Twin Earth initially refers to water on Earth, over time it comes to denote XYZ. (The switch is thus a “slow” one.) After all, Oscar will acquire the habit of using ‘water’ in the presence of XYZ, and of using ‘water’ to converse with Twin Earthians about the stuff. Yet if ‘water’ changes its referent, then (by the earlier Frege-Carnap view) this signals a shift in intension or content. And the change in content can occur without Oscar noticing. Indeed it seems Oscar cannot detect the switch just from the armchair.