Tuesday, March 3, 2009

The Mechanization of the Mind

I read a good book over the holidays: The Mechanization of the Mind, by the French philosopher Jean-Pierre Dupuy. (Coming out in paperback at the end of May, if you're wondering what to get me for my birthday. ;-)

It's a history of Cybernetics, the early 20th century intellectual movement that gave rise to contemporary cognitive science (as well as various other fields like information theory, artificial intelligence, and analytic philosophy of mind). Thus it offers a kind of recent pre-history of today's prevalent assumptions about the nature of the mind and the brain.

The most interesting thing I got out of the book was the fact that the computational theory of mind did not, as I had always assumed, arise as a consequence of the invention of the digital computer. That is, it's not the case that we first invented computers, and then started using them as a metaphor or model for the way our own minds work. As it turns out, the modern digital computer and the computational view of mind share a common origin in mathematical logic.

Here's how it all went down, according to Dupuy.

1. Frege et al. invent modern mathematical logic, which attempts to give a purely syntactic (i.e. symbolic, algorithmic, formal, "mechanical") account of logical inference.

2. The power of this new logic, along with the view that logic prescribes the "laws of thought", lead to the claim that thinking just is this sort of formal symbol-manipulation. Logicism is born.

2. Alan Turing, in his paper "On Computable Numbers, With an Application to the Entscheidungsproblem", tries to give a purely syntactic definition of logical inference by describing an imaginary machine that could write and erase symbols on an infinitely long tape, and "remember" the symbols it has recently scanned in a finite "memory". This machine scans up and down the tape, modifying it in response to the symbols it finds.

While the formal inference rules of symbolic logic had been described as "mechanical" before, in the sense of proceeding without understanding or insight, this was the first time anyone had proposed the idea that these formal inferences could be carried out by an actual machine.

In fact, Turing's machine could only be imaginary, since its tape was infinitely long. But it's not hard to see how a similar machine could be constructed with a finite "tape" - and indeed, it wasn't long before John Von Neumann proposed such a machine. And so the computer as we know it was born: a machine whose instructions are stored in its own memory, and so can be modified by the machine's own activity; a machine in which hardware and software can be distinguished.

4. Since Logicism had already identified thinking with the symbol-manipulation of formal logic, two conclusions seemed inevitable:
a) Machines are capable of thought;
b) The human brain is itself -- or at least, can be adequately modelled by -- a Turing/Von Neumann machine, a computer with a very complex program.

Thus, the computational theory of mind was not the consequence, but rather the antecedent of the invention of the modern computer. The idea that thought consisted in the purely formal manipulation of symbols gave rise more or less simultaneously to the idea that a machine might be able to think, and that the human brain must be such a machine.