I begin my presentation of Hilary Putnam's paper "Brains in a Vat" by sketching Putnam's strategy in large, cartoonlike strokes. Like Bostrom's paper, Putnam's engages in a form of radical speculation, albeit in a different and more sophisticated way. The best way to bring out what is radical in Putnam's program is to caricature it in a way that puts its radicalism into relief. Fundamentally, the radicalism of Putnam's program is associated with his claim that philosophy can do a sort of work of that physics, or more generally any natural science, is incapable of doing. To insist on this point, Putnam has to uphold a distinction between logical and practical possibility—or something like that. I begin by emphasizing the way he does this.