JMC@SAIL.STANFORD.EDU (John McCarthy) (06/22/88)
Date: Mon, 20 Jun 88 06:22 EDT From: John McCarthy <JMC@SAIL.Stanford.EDU> Subject: Ding an sich To: ailist@AI.AI.MIT.EDU I want to defend the extreme point of view that it is both meaningful and possible that the basic structure of the world is unknowable. It is also possible that it is knowable. It just depends on how much of the structure of the world happens to interact with us. This is like Kant's "Ding an sich" (the thing in itself) except that I gather that Kant considered "Ding an sich" as unknowable in principle, whereas I only consider that it might be unknowable. The basis of this position is the notion of evolution of intelligent beings in a world not created for their scientific convenience. There is no mathematical theorem requiring that if a world evolves intelligent beings, these beings must be in a position to discover all its laws. To illustrate this idea, consider the Life cellular automaton proposed by John Horton Conway and studied by him and various M.I.T. hackers and others. It's described in Winning Ways by Berlekamp, Conway and Guy. Associated with each point of the two dimensional integer lattice is a state that takes values 0 and 1. The state of a point at time t+1 is determined by its state at time t and the states at time t of its eight neighbors. Namely, if the number of neigbors in state 1 is less than two or more than 4, its state at time t+1 is 0. If it has exactly two neighbors in state 1, its state remains as it was. If it has exactly 3 neighbors in state 1, its new state is 1. There is a configuration of five cells in state 1 (with neighbors in state 0) called a glider, which reproduces itself displaced in two units of time. There is a configuration called a glider gun that emits gliders. There are configurations that thin out streams of gliders from a glider gun. There are configurations that take two streams of gliders as inputs and perform logical operations (regarding the presence of a glider at a given time in the stream as 1 and its absence as 0) on them producing a new stream. Thinned streams can cross each other and serve as wires conducting signals. This permits the construction of general purpose computers in the Life plane. The Life automaton wasn't designed to admit computers. The discovery that it did was made by hacking. Configurations that can serve as general purpose computers can be made in a variety of ways. The way indicated above and more fully described in Berlekamp, et. al. is only one. Now suppose that one or more interacting Life computers are programmed to be physicists, i.e. to attempt to discover the fundamental physics of their world. There is no reason to expect a mathematical theorem about cellular automata in general or the Life cellular automaton in particular that says that a physicist program will be able to discover that the fundamental physics of its world is the Life cellular automaton. It requires some extra attention in the design of the computer to make sure that it has any capability to observe at all, and some that can observe will be unable to observe enough detail. Of course, we could program a Life computer to simulate some other "second level" cellular automaton that admits computers, and give the "second level computer" only the ability to observe the "second level world". In that case, it surely couldn't find any evidence for the its world being the Life cellular automaton. Indeed the Life automaton could simulate exceedingly slowly any theory we like of our 3+1 dimensional world. If a Life world physicist is provided with too narrow a philosophy of science, and some of the consensual reality theories may indeed be that narrow, it might not regard the hypothesis that its physics is the Life world as meaningful. There may be Life world physicists who regard it as meaningful and Life world philosophers of science interacting with them who try to forbid it. This illustrates what I mean by metaepistemology. Metaepistemology must study what knowledge is possible for intelligent beings in a world to the structure of the world and the physical structures and computational programs that support scientific activity. The traditional methods of philosophy of science are too weak to discuss these matters, because they don't take into account how the structure of the world and the structure of its intelligences affect what science is possible. There is no more guarantee that the structure of our world is observable than that Fermat's last theorem is decidable in Peano arithmetic. Physicists are always proposing theories of fundamental physics whose testability depends on the correctness of other theories and the development of new apparatus. For example, some of the current GUTS theories predict unification of the force laws at energies of 10^15 Mev, and there is no current idea of how an accelerator producing such an energy might be physically possible. I have received messages asking me if the metaepistemology I propose is like what has been proposed by Kant and other philosophers or even by Winograd and Flores. As far as I can tell it's not, and all those mentioned are subject to the criticism of the previous paragraph.