The Turing Principle

Definition

Deutsch takes Alan Turing's abstract mathematical result about universal computation and reframes it as an empirical law of physics: "It is possible to build a universal computer" — one that, given enough time and memory, can be programmed to simulate any other physical process to arbitrary accuracy. Unlike Turing's original formulation (an abstract machine with an infinite tape), Deutsch insists the principle is about what real, buildable hardware in our universe can do — which makes it a testable claim about physics itself, alongside the laws of thermodynamics, not a theorem confined to pure mathematics.

In the Book

Chapter 6 gets there by pushing the universal image generator of Chapter 5 further: a universal virtual-reality generator needs not just unlimited storage (arrangeable in principle) but unlimited computational speed relative to whatever it renders — and the way around that limit is to slow down the user's own information-processing (in the limiting case, the user's brain) to match. Deutsch stresses the principle is not just about classical computers: quantum computers, whose existence he argues follows from taking the multiverse seriously, can perform some computations no classical universal computer can efficiently replicate, which he treats as direct experimental evidence that computation is a physical, not purely mathematical, phenomenon — its power depends on which laws of physics actually hold. He returns to the principle repeatedly: in Chapter 8 he argues that life itself is essentially virtual-reality rendering by genes ("genes embody knowledge about their niches"), and that the Turing principle is what makes this kind of knowledge-embodying process physically possible at all; in Chapter 14 the same principle underwrites the far-future omega-point scenario, where computation could continue for a subjectively infinite time even as the physical universe collapses in a finite one.

Why It Matters

Treating universality of computation as a physical law rather than a mathematical curiosity changes what counts as evidence for or against it — you test it the way you test any law of nature, by seeing whether attempts to build ever-more-general computing machines keep succeeding, not by proving theorems. It also licenses a specific form of argument Deutsch uses repeatedly: if some phenomenon (life, mind, knowledge-growth) turns out to require universal computation to exist, that phenomenon inherits the same footing as a law of physics, rather than being dismissed as an accident of biology or history. That move — grounding an emergent, high-level capability in a stated physical principle rather than treating it as brute fact — is a template for connecting computation, cognition, and physical law in any domain that leans on "in principle, it's just computation."