1. The actuality of infinity
is a paramount metaphysical issue.
Some major issues in science and philosophy demand taking a
position on whether there can be an infinite number of things or an infinite
amount of something. Infinity’s most obvious scientific relevance is to
cosmology, where the question of whether the universe is finite or infinite
looms large. But infinities are invoked in various physical theories, and
they seem often to occur in dubious theories. In quantum mechanics, an
(uncountable) infinity of worlds is invoked by the “many worlds interpretation,”
and anthropic explanations often invoke an actual infinity of universes, which
may themselves be infinite. These applications make actual infinite sets a
paramount metaphysical problem—if it indeed is metaphysical—but the orthodox
view is that, being empirical, it isn’t metaphysical at all. To view infinity
as a purely empirical matter is the modern view; we’ve learned not to place
excessive weight on purely conceptual reasoning, but whether
conceptual reasoning can definitively settle the matter differs from whether the
matter is fundamentally conceptual.
Two developments have discouraged the metaphysical exploration
of actually existing infinities: the mathematical analysis of infinity and the proffer
of crank arguments against infinity in the service of retrograde causes.
Although some marginal schools of mathematics reject Cantor’s investigation of transfinite
numbers, I will assume the concept of infinity itself is consistent. My
analysis pertains not to the concept of infinity as such but to the actual
realization of infinity. Actual infinity’s main detractor is a Christian
fundamentalist crank named William Lane Craig, whose critique of infinity, serving
theist first-cause arguments, has made infinity eliminativism intellectually disreputable.
Craig’s arguments merely appeal to the strangeness of infinity’s
manifestations, not to the incoherence of its realization. The standard arguments against infinity,
which predate Cantor, have been well-refuted, and I leave the mathematical
critique of infinity to the mathematicians, who are mostly satisfied. (See Graham
Oppy, Philosophical perspectives on
infinity (2006).)
2. The principle of
the identity of indistinguishables applies to physics and to actual sets, not to
everything conceivable.
My novel arguments are based on a revision of a metaphysical
principle called the identity of
indistinguishables, which holds that two separate things can’t have exactly
the same properties. Things are constituted by their properties; if two things
have exactly the same properties, nothing remains to make them different from
one another. Physical objects do seem to conform to the identity of
indistinguishables because physical objects are individuated by their positions
in space and time, which are properties, but this is a physical rather than a
metaphysical principle. Conceptually, brute distinguishability, that is, differing from all other things simply in
being different, is a property, although
it provides us with no basis for identifying one thing and not another.
There may be no way to use such a property in any physical theory, we may never
learn of such a property and thus never have reason to believe it instantiated,
but the property seems conceptually possible.
But the identity of indistinguishables does apply to sets of existing things (actual sets): indistinguishable
actual sets are identical. Properties determine actual sets, so you can’t define a proper subset of brutely distinguishable things.
3. Arguments against
actual infinite sets.
A. Argument based on
brute distinguishability.
To show that the existence of an actual infinite set leads to contradiction, assume the existence of an
infinite set of brutely distinguishable points. Now another point pops into
existence. The former and latter sets are indistinguishable, yet they aren’t
identical. The proviso that the points themselves are
indistinguishable allows the sets to be different yet indistinguishable when
they’re infinite, proving they can’t be infinite.
B. Argument based on
probability as limiting relative frequency.
The previous argument depends on the coherence of brute distinguishability. The following probability argument depends on different intuitions. Probabilities can be
treated as idealizations at infinite limits. If you toss a coin, it will land
heads roughly 50% of the time, and it gets closer to exactly 50% as the number
of tosses “approaches infinity.” But if there can actually be an infinite number
of tosses, contradiction arises. Consider the possibility that in an infinite
universe or an infinite number of universes, infinitely many coin tosses
actually occur. The frequency of heads and of tails is then infinite,
so the relative frequency is undefined. Furthermore, the frequency of rolling a
1 on a die also equals the frequency of rolling 2 – 6: both are (countably)
infinite. But when there are infinitely many occurrences, relative frequency should equal the probability approached in a finite world. Therefore, infinite quantities
don’t exist.
4. The nonexistence
of actually realized infinite sets and the principle of the identity of
indistinguishable sets together imply the Gold model of the cosmos.
Before applying the conclusion that actual infinite sets can’t exist, together with the principle of the identity of
indistinguishables, to a fundamental problem of cosmology, caveats are in
order. The argument uses only the most general and well-established physical
conclusions and is oblivious to physical detail, and not being competent in
physics, I must abstain even from assessing the weight the philosophical
analysis that follows should carry; it may be very slight. While the
cosmological model I propose isn’t original, the argument is original and as
far as I can tell, novel. I am not proposing a physical theory as much as
suggesting metaphysical considerations that might bear on physics, whereas it
is for physicists to say how weighty these considerations are in light of
actual physical data and theory.
The cosmological theory is the Gold model of the universe, once favored by Albert Einstein, according to which the universe undergoes a
perpetual expansion, contraction, and re-expansion. I assume a deterministic
universe, such that cycles are exactly identical: any contraction is thus
indistinguishable from any other, and any expansion is indistinguishable from
any other. Since there is no room in physics for brute distinguishability,
they are identical because no common spatio-temporal framework allows their distinction. Thus, although the expansion
and contraction process is perpetual, it is also finite; in fact, its
number is unity.
The Gold universe—alone, with the possible exception
of the Hawking universe—avoids the dilemma of the realization of infinite sets or
origination ex nihilo.
(Edited July 25, 2013: Clarified in Section 2, last paragraph and other places, that the identity of indistinguishables applies to actual sets.)