Microcosm and Macrocosm

David Pears

  • Reason, Truth and History by Hilary Putnam
    Cambridge, 222 pp, £15.00, February 1982, ISBN 0 521 23035 7

There is an odd experience that Plato may have had. If light filters into a room through a small enough aperture, anything moving on the street outside will cast its shadow on the ceiling and back wall, and the shadow may have only the most abstract resemblance to the original. Perhaps the human predicament is really like that. The truth about the world may be difficult or even impossible to attain by ordinary methods.

The way to develop this kind of speculation is to take a part of human experience that is known to provide us with inadequate representations and to suggest that the whole may really be inadequate in a similar way. A skull is quite like a camera obscura and Plato’s Cave is a powerful image. Hilary Putnam’s version of the speculation is more frightening. He imagines brains in a vat of nutrient liquid with a computer giving their sensory nerve-sockets all the stimulations of daily life and collecting and taking account of all the impulses coming from their motor-nerve outlets.

Plato’s prisoners could escape from the cave, but Putnam supposes that his brains in a vat will never find their way into skulls placed on the shoulders of bodies that walk the world. Is there any other way in which they might discover their predicament? If not, then, given that the microcosm of their experiences is exactly like ours, how can we tell that ours is not produced in the same way as theirs? So how do we know that we attain the objective truth about the macrocosm?

The theme of Putnam’s book is that truth is attainable only if it is partly dependent on the mind and so partly constituted by the methods by which it is attained. If this is a viable strategy for avoiding scepticism, it is one that needs careful planning, because it is easy to go too far and to fall into subjectivism.

The speculation that we might be brains in a vat seems to be endemic at Harvard. Robert Nozick, in his recent book, argues ingeniously that, though he does not know that it is false, he does know something incompatible with it – namely, that he is in Cambridge, Massachusetts. Putnam does not let things get so far, because he thinks that the speculation is necessarily false. His argument is that, if the brains in a vat described their predicament in our vocabulary, the words would have to be interpreted not in our way but in theirs. The two nouns, ‘brains’ and ‘vat’, as used by them (impulses picked up by the computer from their motor-nerve outlets), would refer, not to the two types of thing to which they refer on our lips, but only to two types of stimulation of their sensory nerve sockets or to related phantasmata in their microcosms. He then claims that, on this interpretation, their description of their predicament could not be true, because (to put the reason in our way) it would imply that they were hallucinating that they were in a vat, and not that they were really in one. So if we actually are in this predicament, our description of it must be false.

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