The Fabric of Reality David Deutch

part of an autonomous external reality, or were they figments of his
imagination? In the latter case he would have to conclude that ‘his
imagination’ was itself a vast, complex, autonomous universe. The same
dilemma confronted the solipsistic professor who, if pressed for
explanations, would be forced to take a position on the nature of the
audience. And the Inquisition would have had to take a position on the
source of the underlying regularity in the motion of planets, a regularity that
is explicable only by reference to the heliocentric theory. For all these
people, taking their own position seriously as an explanation of the world
would lead them directly to realism and Galilean rationality.
But Dr Johnson’s idea is more than a refutation of solipsism. It also
illustrates the criterion for reality that is used in science, namely, 
if something
can kick back, it exists. ‘Kicking back’ here does not necessarily mean that
the alleged object is responding to being kicked — to being physically
affected as Dr Johnson’s rock was. It is enough that when we ‘kick’
something, the object affects us in ways that require independent
explanation. For example, Galileo had no means of affecting planets, but he
could affect the light that came from them. His equivalent of kicking the rock
was refracting that light through the lenses of his telescopes and eyes. That
light responded by ‘kicking’ his retina back. The way it kicked back allowed
him to conclude not only that the light was real, but that the heliocentric
planetary motions required to explain the patterns in which the light arrived

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were also real.
By the way, Dr Johnson did not directly kick the rock either. A person is a
mind, not a body. The Dr Johnson who performed the experiment was a
mind, and that mind directly ‘kicked’ only some nerves, which transmitted
signals to muscles, which propelled his foot towards the rock. Shortly
afterwards, Dr Johnson perceived being ‘kicked back’ by the rock, but again
only indirectly, after the impact had set up a pressure pattern in his shoe,
and then in his skin, and had then led to electrical impulses in his nerves,
and so forth. Dr Johnson’s mind, like Galileo’s and everyone else’s, ‘kicked’
nerves and ‘was kicked back’ by nerves, and inferred the existence and
properties of reality from those interactions alone. 
What Dr Johnson was
entitled to infer about reality depends on how he could best explain what had
happened. For example, if the sensation had seemed to depend only on the
extension of his leg, and not on external factors, then he would probably
have concluded that it was a property of his leg, or of his mind alone. He
might have been suffering from a disease which gave him a rebounding
sensation whenever he extended his leg in a certain way. But in fact the
rebounding depended on what the rock did, such as being in a certain place,
which was in turn related to other effects that the rock had, such as being
seen, or affecting other people who kicked it. Dr Johnson perceived these
effects to be autonomous (independent of himself) and quite complicated.
Therefore the realist explanation of why the rock produces the rebounding
sensation involves a complicated story about something autonomous. But so
does the solipsist explanation. In fact, any explanation that accounts for the
foot-rebounding phenomenon is necessarily a ‘complicated story about
something autonomous’. It must in effect be the story of the rock. The
solipsist would call it a dream-rock, but apart from that claim the solipsist’s
story and the realist’s could share the same script.
My discussion of shadows and parallel universes in Chapter 2 revolved
around questions of what does or does not exist, and implicitly around what
should or should not count as evidence of existence. I used Dr Johnson’s
criterion. Consider again the point X on the screen in Figure 2.7 (p. 41),
which is illuminated when only two slits are open but goes dark when two
further slits are opened. I said that it is an ‘inescapable’ conclusion that
something must be coming through the second pair of slits to prevent the
light from the first pair from reaching X. It is not 
logically inescapable, for if
we were not looking for explanations we could just say that the photons we
see behave 
as if something passing through other slits had deflected them,
but that in fact there is nothing there. Similarly, Dr Johnson could have said
that his foot rebounded 
as if a rock had been there, but that in fact there was
nothing there. The Inquisition did say that the planets were seen to move as
if they and the Earth were in orbit round the Sun, but that in fact they moved
round the fixed Earth. But if the object of the exercise is to explain the
motion of planets, or the motion of photons, we must do as Dr Johnson did.
We must adopt a methodological rule that if something behaves as if it
existed, by kicking back, then one regards that as evidence that it does exist.
Shadow photons kick back by interfering with the photons that we see, and
therefore shadow photons exist.
Can we likewise conclude from Dr Johnson’s criterion that ‘planets move as
if they were being pushed by angels; therefore angels exist’? No, but only

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because we have a better explanation. The angel theory of planetary motion
is not 
wholly without merit. It does explain why planets move independently
of the celestial sphere, and that does indeed make it superior to solipsism.
But it does not explain why the angels should push the planets along one set
of orbits rather than another, or, in particular, why they should push them as
if their motion were determined by a curvature of space and time, as
specified in every detail by the universal laws of the general theory of
relativity. That is why the angel theory cannot compete as an explanation
with the theories of modern physics.
Similarly, to postulate that angels come through the other slits and deflect
our photons would be better than nothing. But we can do better than that.
We know exactly how those angels would have to behave: very much like
photons. So we have a choice between an explanation in terms of invisible
angels pretending to be photons, and one in terms of invisible photons. In
the absence of an independent explanation for why angels should pretend to
be photons, that latter explanation is superior.
We do not feel the presence of our counterparts in other universes. Nor did
the Inquisition feel the Earth moving beneath their feet. And yet, it moves!
Now, consider what it would feel like if we did exist in multiple copies,
interacting only through the imperceptibly slight effects of quantum
interference. This is the equivalent of what Galileo did when he analysed
how the Earth would feel to us if it were moving in accordance with the
heliocentric theory. He discovered that the motion would be imperceptible.
Yet perhaps ‘imperceptible’ is not quite the right word here. Neither the
motion of the Earth nor the presence of parallel universes is directly
perceptible, but then neither is anything else (except perhaps, if Descartes’s
argument holds, your own bare existence). But both things 
are perceptible in
the sense that they perceptibly ‘kick back’ at us if we examine them through
scientific instruments. We can see a Foucault pendulum swing in a plane
that gradually seems to turn, revealing the rotation of the Earth beneath it.
And we can detect photons that have been deflected by interference from
their other-universe counterparts. It is only an accident of evolution, as it
were, that the senses we are born with are not adapted to feel such things
‘directly’.
It is not how hard something kicks back that makes the theory of its
existence compelling. What matters is its role in the explanations that such a
theory provides. I have given examples from physics where very tiny ‘kicks’
lead us to momentous conclusions about reality because we have no other
explanation. The converse can also happen: if there is no clear-cut winner
among the contending explanations, then even a very powerful ‘kick’ may
not convince us that the supposed source has independent reality. For
example, you may one day see terrifying monsters attacking you — and then
wake up. If the explanation that they originated within your own mind seems
adequate, it would be irrational for you to conclude that there really are such
monsters out there. If you feel a sudden pain in your shoulder as you walk
down a busy street, and look around, and see nothing to explain it, you may
wonder whether the pain was caused by an unconscious part of your own
mind, or by your body, or by something outside. You may consider it
possible that a hidden prankster has shot you with an air-gun, yet come to
no conclusion as to the reality of such a person. But if you then saw an air-

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gun pellet rolling away on the pavement, you might conclude that no
explanation solved the problem as well as the air-gun explanation, in which
case you would adopt it. In other words, you would tentatively infer the
existence of a person you had not seen, and might never see, just because
of that person’s role in the best explanation available to you. Clearly the
theory of such a person’s existence is not a logical consequence of the
observed evidence (which, incidentally, would consist of a single
observation). Nor does that theory have the form of an ‘inductive
generalization’, for example that you will observe the same thing again if you
perform the same experiment. Nor is the theory experimentally testable:
experiment could never prove the absence of a hidden prankster. Despite all
that, the argument in favour of the theory could be overwhelmingly
convincing, if it were the best explanation.
Whenever I have used Dr Johnson’s criterion to argue for the reality of
something, one attribute in particular has always been relevant, namely
complexity. We prefer simpler explanations to more complex ones. And we
prefer explanations that are capable of accounting for detail and complexity
to explanations that can account only for simple aspects of phenomena. Dr
Johnson’s criterion tells us to regard as real those complex entities which, if
we did 
not regard them as real, would complicate our explanations. For
instance, we must regard the planets as real, because if we did not we
should be forced into complicated explanations of a cosmic planetarium, or
of altered laws of physics, or of angels, or of whatever else would, under that
assumption, be giving us the illusion that there are planets out there in
space.
Thus the observed complexity in the structure or behaviour of an entity is