The Fabric of Reality David Deutch

part of the Earth’s surface, on every clear night, for billions of years, has
been deluged with evidence about the facts and laws of astronomy. For
many other sciences evidence has similarly been on display, to be viewed
more clearly in modern times by microscopes and other instruments. Where

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evidence is not already physically present, we can bring it into existence with
devices such as lasers and pierced barriers — devices which it is open to
anyone, anywhere and at any time, to build. And the evidence will be the
same, regardless of who reveals it. The more fundamental a theory is, the
more readily available is the evidence that bears upon it (to those who know
how to look), not just on Earth but throughout the multiverse.
Thus physical reality is 
self-similar on several levels: among the stupendous
complexities of the universe and multiverse, some patterns are nevertheless
endlessly repeated. Earth and Jupiter are in many ways dramatically
dissimilar planets, but they both move in ellipses, and they are made of the
same set of a hundred or so chemical elements (albeit in different
proportions), and so are their parallel-universe counterparts. The evidence
that so impressed Galileo and his contemporaries also exists on other
planets and in distant galaxies. The evidence being considered at this
moment by physicists and astronomers would also have been available a
billion years ago, and will still be available a billion years hence. The very
existence of general, explanatory theories implies that disparate objects and
events are physically alike in some ways. The light reaching us from distant
galaxies is, after all, only light, but it looks to us like galaxies. Thus reality
contains not only evidence, but also the means (such as our minds, and our
artefacts) of understanding it. There 
are mathematical symbols in physical
reality. The fact that it is we who put them there does not make them any
less physical. In those symbols — in our planetariums, books, films and
computer memories, and in our brains — there are images of physical reality
at large, images not just of the appearance of objects, but of the structure of
reality. There are laws and explanations, reductive and emergent. There are
descriptions and explanations of the Big Bang and of subnuclear particles
and processes; there are mathematical abstractions; fiction; art; morality;
shadow photons; parallel universes. To the extent that these symbols,
images and theories are true — that is, they resemble in appropriate
respects the concrete or abstract things they refer to — their existence gives
reality a new sort of self-similarity, the self-similarity we call knowledge.
TERMINOLOGY
heliocentric theory The theory that the Earth moves round the Sun, and
spins on its own axis.
geocentric theory The theory that the Earth is at rest and other astronomical
bodies move around it.
realism The theory that an external physical universe exists objectively and
affects us through our senses.
Occam’s razor (My formulation) 
Do not complicate explanations beyond
necessity, because if you do, the unnecessary complications themselves will
remain unexplained.
Dr Johnson’s criterion (My formulation) 
If it can kick back, it exists. A more
elaborate version is: 
If, according to the simplest explanation, an entity is
complex and autonomous, then that entity is real.

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 self-similarity Some parts of physical reality (such as symbols, pictures or
human thoughts) resemble other parts. The resemblance may be concrete,
as when the images in a planetarium resemble the night sky; more
importantly, it may be abstract, as when a statement in quantum theory
printed in a book correctly explains an aspect of the structure of the
multiverse. (Some readers may be familiar with the geometry of fractals; the
notion of self-similarity defined here is much broader than the one used in
that field.)
complexity theory The branch of computer science concerned with what
resources (such as time, memory capacity or energy) are required to
perform given classes of computations.
SUMMARY
Although solipsism and related doctrines are logically self-consistent, they
can be comprehensively refuted simply by taking them seriously as
explanations. Although they all claim to be simplified world-views, such an
analysis shows them to be indefensible over-elaborations of realism. Real
entities behave in a complex and autonomous way, which can be taken as
the criterion for reality: if something ‘kicks back’, it exists. Scientific
reasoning, which uses observation not as a basis for extrapolation but to
distinguish between otherwise equally good explanations, can give us
genuine knowledge about reality.
Thus science and other forms of knowledge are made possible by a special
self-similarity property of the physical world. Yet it was not physicists who
first recognized and studied this property: it was mathematicians and
computer theorists, and they called it the universality of computation. The
theory of computation is our third strand.

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