Theoretical physicist David Deutsch states that the Turing principle (an extrapolation from the Church-Turing conjecture) is "what makes reality comprehensible". A grand claim, but one that is explicable.

Deutsch initially states the Turing principle as follows:

There exists an abstract universal computer whose repertoire includes any computation that any physically possible object can perform

Deutsch's use of the phrase "abstract computer" allows for the use of quantum computers, which sidesteps many of the limits of current computing technology.

Deutsch then states that a genuine universal computer must be a physically real object, since a purely abstract computer is not limited by what is or is not computable. As such, Deutsch rephrases the Turing principle:

It is possible to build a universal computer: a machine that can be programmed to perform any computation that any other physical object can perform.

This rephrasing of the principle increases a universal computer's abilities; whereas before an abstract universal computer could simulate any physical object, now a physical universal computer can simulate any other physical universal computer.

Deutsch now proposes connecting our universal computer (capable of simulating any physical phenomenon) to what he calls a "universal image generator"; a machine capable of stimulating a human brain in such a way as to simulate, within a human mind, any environment one can program into the generator. The current technological unfeasibility of such a machine is set aside by the acknowledgement that creating one will eventually be possible.

The joining of the universal computer with the universal image generator now allows us to simulate, within the human mind, anything physically possible. Deutsch labels this creation a "universal virtual-reality generator", and rephrases the Turing principle again to support it:

It is possible to build a virtual-reality generator whose repertoire includes that of every other physically possible virtual-reality generator.

This phraseology can be simplified, if one accepts that the universal virtual-reality generator can render any physically possible environment, to the following:

It is possible to build a virtual-reality generator whose repertoire includes every physically possible environment.

In essence, this form of the Turing principle tells us that the whole of physical reality can be described by a vastly smaller part of that reality. This property of the universe (or multiverse) is called self-similarity.

It is the property of self-similarity which, Deutsch argues, allows the human mind to understand anything. The human mind itself is a virtual-reality generator which creates a simulation of physical reality from sensory data.

Here, in Deutsch's words, is the summation:

If the laws of physics as they apply to any physical object or process are to be comprehensible, they must be capable of being embodied in another physical object - the knower. It is also necessary that processes capable of creating such knowledge be physically possible. Such processes are called science. Science depends on experimental testing, which means physically rendering a law's predictions and comparing it with (a rendering of) reality. It also depends on explanation, and that requires the abstract laws themselves, not merely their predictive content, to be capable of being rendered in virtual reality...The laws of physics, by conforming to the Turing principle, make it physically possible for those same laws to become known to physical objects. Thus, the laws of physics may be said to mandate their own comprehensibility.

The above consists of both my (probably poor) explanation of some of David Deutsch's theories, and quotations from his book The Fabric of Reality. I would request that those who understand his ideas better than I do contribute to this node.

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