 Putnam’s argument comes in Chapter 1 of Reason, Truth and History, where his goal is to refute a modern-day version of Descartes’ evil demon hypothesis, according to which I do not inhabit a world containing ordinary physical objects (trees, tables, houses), but I am a brain in a vat in a lab whose experiences are caused by a computer artificially stimulating my nerve endings, so that none of these objects actually exist beyond my hallucinatory impression of them. This is the brain-in-a-vat hypothesis, and it stands for the possibility that, for all I know, nothing rules out the world being very different from how it appears to me to be, given the gap that exists between appearance (our experience of it) and reality.