Substrates part 1
The term "substrate" comes up often in discussions about AI, especially in the context of comparing AI to biological intelligence. The meaning of the term is pretty straightforward: the substrate is the hardware that embodies a particular intelligence. Human intelligence, then, runs on a meat substrate: all the neurons that make up the human nervous system, along with their support infrastructure. In traditional computing, we normally use the term "platform" to refer to the same concept. For example, I'm writing this article in Word (the software), which is running on an Intel Core2 / Windows Vista computer (the platform, or substrate).
One of the earliest theoretical results of computer science was Alan Turing's development of an imaginary computing platform called the Turing Machine. A Turning Machine is about as primitive as a computer can get: it consists of a very long paper tape with numbers written on it and a mechanical head that moves up and down the tape reading and writing numbers according to some simple rules. Despite the rudimentary nature of the Turing Machine, Turing was able to prove a strange and profoundly important theorem:
Aside from speed and memory constraints, all possible computers are functionally identical to a Turing Machine, and therefore to each other.
One real-world implication of this is that it's possible for one computer to exactly emulate another. You've seen this in action if you've ever run an emulator for an old computer game. MAME (Multiple Arcade Machine Emulator) is an example of this: it's a program that allows modern computers to run games that were originally written for old video arcade machines. Even though Space Invaders was written to run on a specific computer system (an old stand-up arcade machine), it can be run on any modern computer for which a version of MAME exists (which is pretty much all of them). This is true even though modern computers are vastly different from 1980s arcade machines, and even though the authors of MAME may have no idea how Space Invaders actually works.
What this means for AI, of course, is that if the human brain is a Turing Machine, it's possible to run its software (ie, human intelligence) on any sufficiently powerful computer, regardless of how greatly its underlying architecture may differ from that of the human brain. In part 2, I'll discuss why the brain is probably a Turing Machine, and explore some of the non-obvious implications of that fact.