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The Uniqueness of the Universe
The claim that the universe is fine-tuned for life has prompted considerable metaphysical
speculation recently. Some theists see in it fodder for a new design argument. Some
atheists respond that fine-tuning is explicable in terms of the existence of a vast multitude
of other universes.i Yet there is a long history of philosophers seeing such metaphysical
speculation as inherently misguided. The universe, they say, is unique. As such, it makes
no sense to talk of the cause of it, the explanation for it, or the probability of its existing.
What are these skeptics getting at? Are their objections any good? I will examine three
versions of the uniqueness objection and conclude that only one raises a serious problem
for efforts to explain fine-tuning metaphysically.
THE CAUSAL VERSION OF THE UNIQUENESS OBJECTION
The causal version of the uniqueness objection to design explanations of the universe is
well-known to those familiar with Hume.
When two species of objects have always been observed to be conjoined
together, I can infer, by custom, the existence of one wherever I see the
existence of the other; and this I call an argument from experience. But
how this argument can have place where the objects, as in the present
case, are single, individual, without parallel or specific resemblance, may
be difficult to explain. And will any man tell me with a serious
countenance that an orderly universe must arise from some thought and art
like the human because we have experience with it? To ascertain this
reasoning it were requisite that we had experience with the origin of
worlds.ii
As J.C.A. Gaskin astutely observes of the passage above, "this is one of the few
occasions in his critique of religion when Hume appeals to a thesis drawn from his own
philosophy."iii In this case, Hume invoked his (positive) theory of causation. According to
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it, there are three general conditions that must be satisfied in order for us to believe that A
causes B: (i) we must believe that A is temporally prior to B; (ii) we must believe that A
is spatially contiguous with B; and (iii) we must expect B to follow A. What makes us
expect B to follow A, Hume says, is that we have observed a "constant conjunction"
between the types of things of which A and B are instances; out of custom or habit (rather
than the recognition of a necessary connection), we come to expect B-type things to
follow A-type things.
In the passage above, Hume's critique of the design argument relies exclusively
on condition (iii).iv For us to think A caused B, we must have (a) observed a (b) constant
conjunction between (c) the types of things of which A and B are instances. Yet to meet
(a) the observational requirement, "it were requisite that we had experience with the
origin of worlds" - presumably an impossibility for creatures like us. Even supposing we
could have experience with the origin of this world, we would have to have experience
with the origin of other worlds as well in order to satisfy (b) the constant conjunction
requirement. And in any case, the universe is "without parallel or specific resemblance."
Since it is not a token of an oft-instantiated type, it fails to meet condition (c).
To those who think the universe is clearly similar to an artifact, Hume argued in
parts V-VIII of Dialogues that it is equally similar to a plant or an animal. Since the
universe no more resembles the members of the class of artifacts than it does the
members of these other classes – since the universe lacks "specific resemblance" – there
is no more reason to think the cause of the universe was artisan-like than to think that the
cause was seed-like or mother-like. In other words, there is no reason to prefer the design
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hypothesis to the hypothesis that the universe was the result of some sort of "generation
and destruction."
Other philosophers have enthusiastically endorsed Hume's objection, with
Anthony Flew saying Hume's point about the uniqueness of the universe delivers to the
design argument a "killing blow."
However far back we may be able to trace the – so to speak – internal
history of the Universe, there can be no question of arguing that this or
that external origin is either probable or improbable. We do not have, and
we necessarily could not have, experience of other Universes to show or
suggest that Universes, or Universes with these particular features, are the
work of Gods, or of Gods of this or that particular sort...[…]...the unique
Universe is and must be itself all we have. How it is, is just how it is; and
that's that.v
Notice Flew says "all we have" (by which he means, I take it, "all we have to go on")
rather than "all there is." Had he said the latter, then it would follow analytically that
there is only one universe. Contemporary physical cosmologists do not use 'universe' this
way; they often speak of other, real universes, especially in connection with the anthropic
principle. It would not affect Hume's point, however, if there were more than one
universe that could exist. His complaint rests simply on the idea that we can only have
experience of one universe.vi
Whether this causal version of the uniqueness objection works depends crucially
on whether the universe is, indeed, "without parallel or specific resemblance." If not – if,
in modern parlance, it can be placed in an appropriate "reference class" – then, contrary
to Hume, we could observe a constant conjunction between the type of thing of which the
universe is an instance and a type of cause. Yet if the universe does lack "specific
resemblance," this is only a contingent fact. In his day Hume may have been warranted in
insisting that the evidence did not justify selecting machine over animal or vegetable as
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that to which the universe bears specific resemblance. As our picture of the universe fills
out, however, the evidence may tilt in favor of some specific analogue.
For example, recent mappings of the galaxies reveal the existence of "foamy"
structures.vii Galaxies are stretched out in stringy filaments, arranged in huge bubbles, and
clustered into gigantic walls with galaxy-free space in between. Some astronomers
interpreting these maps take them as pointing to phase transitions in the early universe.
Part of their reason is that we see the same sorts of features in other substances produced
as a result of phase transitions (e.g. the cracks, streaks, and bubbles that develop as liquid
water turns to solid ice).viii Of course it may turn out that a fuller picture of the universe
leads cosmologists to abandon these analogies – but then again, it may not. The point is
that there is no general reason for supposing that the only sort of thing to which a
universe might properly be compared is another universe. Whether or not the universe
bears a specific resemblance to some familiar kind of object is a thoroughly empirical
matter.
THE EXPLANATORY VERSION OF THE UNIQUENESS OBJECTION
Under the influence of a deductive-nomological (DN) picture of scientific explanation,
some philosophers think those who try to ascribe a cause to the universe are guilty of a
category mistake. They say cosmologists are capable of offering us only a model of the
universe, not a theory. W.H. McCrea illustrated the distinction between models and
theories by contrasting ancient astronomy with modern. For the Greeks, the goal of
astronomy was to develop a model that accurately described the motions of the planets
and enabled future motions to be predicted. Newton, on the other hand, looked for a
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theory from which he could deduce, not just the motions of the planets, but the motions
of any other physical system as well. The Newtonian approach is the ideal contemporary
cosmologists follow as they study the universe, said McCrea, but they would be better off
approaching the universe in the same way that the Greeks approached astronomy.
Now we maintain that Newton did better than the Greeks because his
methods allowed him to deal with any number of gravitating systems as
easily as with the solar system, whereas the Greek method entailed a
repetition of the whole work for every system encountered. On the other
hand, there is but one universe and so we should regard the Greek method
here as the superior one. Of course, we expect some time to find a better
method but not in this case one along the lines of Newtonian theory.ix
McCrea thought the only reason to theorize about the cause of the universe would be to
uncover laws that would explain the existence and nature of other universes as well.
Since there are no other universes (or, at least, none open to our observation), the search
for causal laws is unnecessary. Milton Munitz adopted a similar position.
Insofar as inductive reasoning is the attempt to provide good reasons for
inferring something about unobserved instances, on the basis of observed
instances, it has to do with the inference to further instances of some
regularity or law. However, the problem faced in cosmology is not that of
finding a warrant in experience for establishing laws. This is the task of
ordinary physics. The task of cosmology, rather, is to say something about
[that to which the observed universe belongs]; for this purpose, a model of
the universe needs to be employed. And a model is not a law.x
John Leslie says such people think "basic laws of nature cannot be explained and
therefore cannot need to be explained."xi As he points out, this belief arises from a
conception of explanation according to which "the right way of explaining any natural
law is to reveal it as an instance of some more basic law."xii Assuming 'more basic' means
'applies to a greater range of phenomena', the most basic laws will be those which apply
to all phenomena and hence are incapable of subsumption. The attitude toward these
basic laws is (to paraphrase Flew) that they are what they are, and that's that.
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This objection is compelling only if one thinks DN is the only correct model of
scientific explanation. Yet clearly there are singular explanations – explanations in which
the phenomena explained are not deducible from some conjunction of laws and initial
conditions – that count as fully scientific. Such explanations are common in "historical"
sciences such as archaeology and evolutionary biology. Furthermore, subsuming
phenomena under a law is not the same thing as providing an explanation – at least if an
explanation is supposed to increase our understanding.
Gregor Mendel identified a pattern regarding the heritability of pea-pod
characteristics, yet arguably he did not explain pea-pod heredity. For one thing, Mendel's
Law is silent about the mechanism of heritability. Providing a mechanism – answering
the “How?” question – is an important aspect of scientific explanation. Primitive human
seafarers were able to uncover the pattern behind the tides, but they had no explanation of
the tides because they did not understand why or how they happened. A view of
explanation according to which subsumption under a law automatically counts as
providing an explanation thus appears to be entirely too easygoing. Non-DN
explanations, on the other hand, are often the best examples of science leading to an
increased understanding of the world. Consider, for example, the theory that the
dinosaurs became extinct as the result of a massive asteroid impact 65 million years ago.
Whatever a scientific explanation is, the asteroid impact theory should count as one. As
with the long-ago extinction event, the origin of the universe, too, is a unique happening,
but that fact alone does not rule out the possibility of explaining it.xiii
THE PROBABILISTIC VERSION OF THE UNIQUENESS OBJECTION
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Contemporary arguments from fine-tuning are almost always couched in probabilistic
terms, with the inference to a designer or to a multiverse cast as a Bayesian one. The
central claim is that the probability on chance of the universe's being such as to permit
life is extraordinarily low. Yet some allege that the uniqueness of the universe precludes
meaningful talk of the probability of the universe's existing and being the way it is. If this
probabilistic version of the uniqueness objection is right, then inferences from fine-tuning
to a designer or to a multiverse will either have to be reformulated in a way that does not
involve probabilities or will have to be abandoned altogether.
The doctrine of the meaninglessness of probability claims about unique events
derives from frequentism. Richard von Mises, the 20th century's foremost proponent of
frequentism, claimed that "the rational concept of probability, which is the only basis of
probability calculus, applies only to problems in which either the same event repeats
itself again and again, or a great number of uniform elements are involved at the same
time."xiv A consequence of this view is that (as Peirce said) "in reference to a single case
considered in itself, probability can have no meaning."xv If the universe is such a single
case, it follows from frequentism that it is meaningless to talk about the probability of the
universe's existing and having the features that it has. As previously noted, the universe
may not be such a case. Perhaps there is some reference class (or "collective," to use von
Mises's term) of observable phenomena into which the universe can naturally be put –
say, the class of foamy physical systems. But what if such a reference class of observable
phenomena cannot be identified?
A tempting view to take – indeed, the standard view in the fine-tuning literature –
is that the reference class is the set of possible universes that follow the same basic laws
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as our universe but in which the values of the free parameters (e.g. the rate of expansion
of the early universe) are different. These possible universes are unobservable, but
knowledgeable physicists can grasp what they would be like (e.g. how conducive they
would be to star formation).
This view suffers from a fundamental problem. Consider a particular free cosmic
parameter P. Though there may be true counterfactuals of the form "If P were slightly
different, then the eventual emergence of life in the universe would be physically
impossible," this would not entail that P's taking a life-permitting value is improbable. A
probability is a number between zero and one. For there to be a probability of a particular
outcome there has to be a space of possible outcomes such that its area or volume adds up
to one. That is, probabilities require normalizable possibility spaces. In an infinite space
in which every value is equally probable, either each particular probability is zero (in
which case they all add up to zero) or each particular probability is nonzero (in which
case their sum is infinite).1 The issue, then, is whether a basis has been provided (either in
physical theory or on philosophical grounds) for restricting the possible values of the
relevant cosmic parameter or for assigning higher probabilities to some of its possible
values than others.xvi Without such a basis, there is no reason to think the space of
possible values for that parameter is normalizable. And in that case, it is meaningless to
talk about the probability of selecting a certain value (or one of a certain set of values)
from that space.xvii Because of this normalizability issue, we cannot cash out claims about
the low probability of the universe's being such as to permit life in terms of the relative
frequency of life-permitting universes in some set of possible universes.
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Things get no better if we work with one of the other standard philosophical
accounts of probability.xviii Subjectivists analyze probability in terms of rational
constraints on betting behavior. The problem, however, is that no sense can be made of
the idea of our collecting on a bet regarding the universe’s having a given set of
constants. [Here we pick up one of the main themes of the uniqueness objector. We are
not angels, capable of peeking in on various universes. We are stuck inside this universe.]
Propensity theory, meanwhile, is inapt because there seems to be nothing in which any
propensity for the production of our universe could be grounded – unless we beg the
question by assuming that there is some concrete reality beyond our universe in which
such a propensity is grounded. That there is such a reality is supposed to be the
conclusion of a design/multiverse argument from fine-tuning, not a premise.
To sum up, the causal and explanatory versions of the uniqueness objection are
weak, whereas the probabilistic version is potentially quite strong. Whether there is an
adequate reply to the latter is a matter for further discussion.xix
i
For careful documentation of the claims of fine-tuning - claims that if various force
strengths, mass ratios, and initial conditions had been slightly different, life could not
possibly have arisen in the universe – see Robin Collins, “Evidence of Fine-tuning,” in
Neil A. Manson (ed.), God and Design: The Teleological Argument and Modern Science,
(London: Routledge, 2003). See also John D. Barrow and Frank J. Tipler, The Anthropic
Cosmological Principle (New York: Oxford University Press, 1986) and John Leslie,
Universes (New York: Routledge, 1989).
ii
David Hume, Dialogues concerning Natural Religion, ed. Nelson Pike (Indianapolis:
Bobbs-Merrill Educational Publishing, 1970) p. 30.
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10
J.C.A. Gaskin, Hume's Philosophy of Religion (New York: Barnes and Noble, 1978),
p. 20.
iv
Interestingly, nowhere in Dialogues does Hume appeal to condition (ii) of his account
of causation when criticizing the design argument. If the universe (or "world") is simply
the totality of all physical things, then there could be no further physical thing spatially
contiguous with it. Perhaps this absence indicates Hume did not define 'world' in this
way. Peter van Inwagen pointed out (correspondence) evidence in favor of this
interpretation. Hume discussed the possibility of a world's having arisen by "vegetation"
from a comet-seed (see Hume, Dialogues, p. 62); this implies that a Humean "world"
could be part of some larger universe that is open to experience.
v
Anthony Flew, David Hume: Philosopher of Moral Science (London: Basil Blackwell,
1986), p. 66.
vi
To say that we can only have experience of one universe is not to say the universe we
inhabit is the only one of which we could have experience. It is logically possible that we
exist in some other universe instead of this one, just as it is logically possible that
Descartes was born in Scotland rather than France. The question of whether (due to the
necessity of material origins) we are metaphysically tied to this particular universe turns
out to be a sticking point with respect to the objection that multiverse explanations of
fine-tuning fail to explain why this universe is fine-tuned. See Roger White, “Fine
Tuning and Multiple Universes,” Nous vol. 34 (2000), pp. 260-76; and Neil A. Manson
and Michael J. Thrush, "Fine-tuning, Multiple Universes, and the 'This Universe'
Objection," Pacific Philosophical Quarterly vol. 84 (2003), pp. 67-83.
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11
See Sidney Perkowitz, Universal Foam: From Cappuccino to the Cosmos (New York:
Walker & Co., 2000) for an entertaining introduction to the physics of foam.
viii
Ivars Peterson reports on these observations in "Seeding the Universe," Science News
vol. 137 (March 24, 1990), pp. 184-7.
ix
W.H. McCrea, "Information and Prediction in Cosmology," The Monist vol. 47, no. 1
(Fall 1962), p. 98.
x
Milton K. Munitz, The Mystery of Existence (New York: New York University Press,
1974), p. 62.
xi
John Leslie, Universes (New York: Routledge, 1989), p. 106.
xii
ibid., p. 109.
xiii
I owe this criticism of the DN account of explanation to Steven Meyer. See his “The
Methodological Equivalence of Design and Descent,” in The Creation Hypothesis, ed.
J.P. Moreland (Downers Grove, IL: InterVarsity Press, 1994), pp. 77-82.
xiv
Richard von Mises, Probability, Statistics, and Truth, 2nd English edition (revised)
(London: George Allen and Unwin, 1957), p. 11.
xv
C.S. Peirce, "The Doctrine of Chances," in Writings of Charles S. Peirce: A
Chronological Edition, vol. 3 (Bloomington, Indiana: Indiana University Press, 1982), p.
281. "The Doctrine of Chances" was published in 1878.
xvi
Some attempts have been made to provide such a basis, but they seem to have failed.
See John Earman and Jesus Mosterin, "A Critical Look at Inflationary Cosmology,"
Philosophy of Science vol. 66 (1999), pp. 31-4. Robin Collins, Alexander Pruss, and
Jeffrey Koperski separately seek to rectify this problem in unpublished papers of theirs.
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12
For a much more detailed presentation of this argument, see Timothy McGrew, Lydia
McGrew, and Eric Vestrup, "Probabilities and the Fine-tuning Argument: A Skeptical
View," Mind vol. 110, no. 440 and Neil A. Manson, "There Is No Adequate Definition of
'Fine-tuned for Life'," Inquiry vol. 43, no. 3 (September 2000). A less formalized
presentation of the same basic idea was given by David Shotwell in "Is the Universe
Improbable?" The Skeptical Inquirer vol. 11 (1987), pp. 376-82.
xviii
For an excellent survey of such accounts, see Donald Gillies, Philosophical Theories
of Probability (New York: Routledge, 2000).
xix
I’d like to thank the participants in the 2002 meeting of the Creighton Club for their
valuable responses to an earlier version of this paper.