The first post in this series examined Hilary Putnam’s famous argument that a “brain in a vat” (BIV) could not know that it was a BIV—or even think or wonder whether it was a BIV—because its words and thoughts would lack the causal-perceptual links to vats and brains in its environment needed for them to refer to those objects. However, as I said in that first post, for Putnam the BIV argument was just a warm-up exercise. He uses the traditional BIV scenario to illustrate what he regards as the key error of “metaphysical realism” (the view that our percepts and thoughts refer to mind-independent things): that it necessarily relies on a God’s Eye perspective from which we can determine what mind-independent things our percepts and thoughts refer to. Of course, there is no God’s Eye perspective available to human beings, and that is why the project of metaphysical realism must end in failure. Thus, Putnam’s real view is that even if the BIV had the same causal-perceptual embedding in its environment that we enjoy, it would make no difference! Its percepts and thoughts would still not refer to mind-independent things. Reference to mind-independent things is impossible in general. The traditional worry about whether you could be a BIV is a useful entrée to these issues because it presupposes metaphysical realism. Only a metaphysical realist would or could worry about being a BIV, because only if the objects of thought were mind-independent would it be possible to be so radically in error about the nature of one’s environment.

Why does Putnam think that only a God’s Eye perspective can determine the reference of our thoughts and percepts? The reason is given in the so-called “model-theoretic argument” that Putnam presents in each of the three works I mentioned in the first post (“Realism and Reason” [R&R], “Models and Reality” [M&R], and Reason, Truth, and History [RT&H]. In the present post, I explain the argument and the “internal realist” view that Putnam advocates on the basis of it. In the next post, we will examine the merits of the model-theoretic argument. (The whole paper on which these posts are based is available here. To skip to the third post in the series, click here.)