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One notable feature of mathematical reasoning is the reliance on counterfactual thinking – taking a hypothesis (or set of hypotheses) which may or may not be true, and following it (or them) to its logical conclusion.  For instance, most propositions in mathematics start with a set of hypotheses (e.g. “Let n be a natural number such that …”), which may or may not apply to the particular value of n one may have in mind.  Or, if one ever argues by dividing into separate cases (e.g. “Case 1: n is even. … Case 2: n is odd.  …”), then for any given n, at most one of these cases would actually be applicable, with the other cases being counterfactual alternatives.     But the purest example of counterfactual thinking in mathematics comes when one employs a proof by contradiction (or reductio ad absurdum) – one introduces a hypothesis that in fact has no chance of being true at all (e.g. “Suppose for sake of contradiction that \sqrt{2} is equal to the ratio p/q of two natural numbers.”), and proceeds to demonstrate this fact by showing that this hypothesis leads to absurdity.

Experienced mathematicians are so used to this type of counterfactual thinking that it is sometimes difficult for them to realise that it this type of thinking is not automatically intuitive for students or non-mathematicians, who can anchor their thinking on the single, “real” world to the extent that they cannot easily consider hypothetical alternatives.  This can lead to confused exchanges such as the following:

Lecturer: “Theorem.  Let p be a prime number.  Then…”

Student: “But how do you know that p is a prime number?  Couldn’t it be composite?”

or

Lecturer: “Now we see what the function f does when we give it the input of x+dx instead.  …”

Student: “But didn’t you just say that the input was equal to x just a moment ago?”

This is not to say that counterfactual thinking is not encountered at all outside of mathematics.  For instance, an obvious source of counterfactual thinking occurs in fictional writing or film, particularly in speculative fiction such as science fiction, fantasy, or alternate history.  Here, one can certainly take one or more counterfactual hypotheses (e.g. “what if magic really existed?”) and follow them to see what conclusions would result.  The analogy between this and mathematical counterfactual reasoning is not perfect, of course: in fiction, consequences are usually not logically entailed by their premises, but are instead driven by more contingent considerations, such as the need to advance the plot, to entertain or emotionally affect the reader, or to make some moral or ideological point, and these types of narrative elements are almost completely absent in mathematical writing.  Nevertheless, the analogy can be somewhat helpful when one is first coming to terms with mathematical reasoning.  For instance, the mathematical concept of a proof by contradiction can be viewed as roughly analogous in some ways to such literary concepts as satire, dark humour, or absurdist fiction, in which one takes a premise specifically with the intent to derive absurd consequences from it.  And if the proof of (say) a lemma is analogous to a short story, then the statement of that lemma can be viewed as analogous to the moral of that story.

Another source of counterfactual thinking outside of mathematics comes from simulation, when one feeds some initial data or hypotheses (that may or may not correspond to what actually happens in the real world) into a simulated environment (e.g. a piece of computer software, a laboratory experiment, or even just a thought-experiment), and then runs the simulation to see what consequences result from these hypotheses.   Here, proof by contradiction is roughly analogous to the “garbage in, garbage out” phenomenon that is familiar to anyone who has worked with computers: if one’s initial inputs to a simulation are not consistent with the hypotheses of that simulation, or with each other, one can obtain bizarrely illogical (and sometimes unintentionally amusing) outputs as a result; and conversely, such outputs can be used to detect and diagnose problems with the data, hypotheses, or implementation of the simulation.

Despite the presence of these non-mathematical analogies, though, proofs by contradiction are still often viewed with suspicion and unease by many students of mathematics.  Perhaps the quintessential example of this is the standard proof of Cantor’s theorem that the set {\bf R} of real numbers is uncountable.  This is about as short and as elegant a proof by contradiction as one can have without being utterly trivial, and despite this (or perhaps because of this) it seems to offend the reason of many people when they are first exposed to it, to an extent far greater than most other results in mathematics.  (The only other two examples I know of that come close to doing this are the fact that the real number 0.999\ldots is equal to 1, and the solution to the blue-eyed islanders puzzle.)

Some time ago on this blog, I collected a family of well-known results in mathematics that were proven by contradiction, and specifically by a type of argument that I called the “no self-defeating object” argument; that any object that was so ridiculously overpowered that it could be used to “defeat” its own existence, could not actually exist.  Many basic results in mathematics can be phrased in this manner: not only Cantor’s theorem, but Euclid’s theorem on the infinitude of primes, Gödel’s incompleteness theorem, or the conclusion (from Russell’s paradox) that the class of all sets cannot itself be a set.

I presented each of these arguments in the usual “proof by contradiction” manner; I made the counterfactual hypothesis that the impossibly overpowered object existed, and then used this to eventually derive a contradiction.  Mathematically, there is nothing wrong with this reasoning, but because the argument spends almost its entire duration inside the bizarre counterfactual universe caused by an impossible hypothesis, readers who are not experienced with counterfactual thinking may view these arguments with unease.

It was pointed out to me, though (originally with regards to Euclid’s theorem, but the same point in fact applies to the other results I presented) that one can pull a large fraction of each argument out of this counterfactual world, so that one can see most of the argument directly, without the need for any intrinsically impossible hypotheses.  This is done by converting the “no self-defeating object” argument into a logically equivalent “any object can be defeated” argument, with the former then being viewed as an immediate corollary of the latter.  This change is almost trivial to enact (it is often little more than just taking the contrapositive of the original statement), but it does offer a slightly different “non-counterfactual” (or more precisely, “not necessarily counterfactual”) perspective on these arguments which may assist in understanding how they work.

For instance, consider the very first no-self-defeating result presented in the previous post:

Proposition 1 (No largest natural number). There does not exist a natural number N that is larger than all the other natural numbers.

This is formulated in the “no self-defeating object” formulation.  But it has a logically equivalent “any object can be defeated” form:

Proposition 1′. Given any natural number N, one can find another natural number N' which is larger than N.

Proof. Take N' := N+1. \Box

While Proposition 1 and Proposition 1′ are logically equivalent to each other, note one key difference: Proposition 1′ can be illustrated with examples (e.g. take N = 100, so that the proof gives N'=101 ), whilst Proposition 1 cannot (since there is, after all, no such thing as a largest natural number).  So there is a sense in which Proposition 1′ is more “non-counterfactual” or  “constructive” than the “counterfactual” Proposition 1.

In a similar spirit, Euclid’s theorem (which we give using the numbering from the previous post),

Proposition 3. There are infinitely many primes.

can be recast in “all objects can be defeated” form as

Proposition 3′.  Let p_1,\ldots,p_n be a collection of primes.   Then there exists a prime q which is distinct from any of the primes p_1,\ldots,p_n.

Proof. Take q to be any prime factor of p_1 \ldots p_n + 1 (for instance, one could take the smallest prime factor, if one wished to be completely concrete).   Since p_1 \ldots p_n + 1 is not divisible by any of the primes p_1,\ldots,p_n, q must be distinct from all of these primes.  \Box

One could argue that  there was a slight use of proof by contradiction in the proof of Proposition 3′ (because one had to briefly entertain and then rule out the counterfactual possibility that q was equal to one of the p_1,\ldots,p_n), but the proposition itself is not inherently counterfactual, as  it does not make as patently impossible a hypothesis as a finite enumeration of the primes.  Incidentally, it can be argued that the proof of Proposition 3′ is closer in spirit to Euclid’s original proof of his theorem, than the proof of Proposition 3 that is usually given today.  Again, Proposition 3′ is “constructive”; one can apply it to any finite list of primes, say 2, 3, 5, and it will actually exhibit a prime not in that list (in this case, 31).  The same cannot be said of Proposition 3, despite the logical equivalence of the two statements.

[Note: the article below may make more sense if one first reviews the previous blog post on the "no self-defeating object".  For instance, the section and theorem numbering here is deliberately chosen to match that of the preceding post.]

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A fundamental tool in any mathematician’s toolkit is that of reductio ad absurdum: showing that a statement {X} is false by assuming first that {X} is true, and showing that this leads to a logical contradiction. A particulary pure example of reductio ad absurdum occurs when establishing the non-existence of a hypothetically overpowered object or structure {X}, by showing that {X}‘s powers are “self-defeating”: the very existence of {X} and its powers can be used (by some clever trick) to construct a counterexample to that power. Perhaps the most well-known example of a self-defeating object comes from the omnipotence paradox in philosophy (“Can an omnipotent being create a rock so heavy that He cannot lift it?”); more generally, a large number of other paradoxes in logic or philosophy can be reinterpreted as a proof that a certain overpowered object or structure does not exist.

In mathematics, perhaps the first example of a self-defeating object one encounters is that of a largest natural number:

Proposition 1 (No largest natural number) There does not exist a natural number {N} which is larger than all other natural numbers.

Proof: Suppose for contradiction that there was such a largest natural number {N}. Then {N+1} is also a natural number which is strictly larger than {N}, contradicting the hypothesis that {N} is the largest natural number. \Box

Note the argument does not apply to the extended natural number system in which one adjoins an additional object {\infty} beyond the natural numbers, because {\infty+1} is defined equal to {\infty}. However, the above argument does show that the existence of a largest number is not compatible with the Peano axioms.

This argument, by the way, is perhaps the only mathematical argument I know of which is routinely taught to primary school children by other primary school children, thanks to the schoolyard game of naming the largest number. It is arguably one’s first exposure to a mathematical non-existence result, which seems innocuous at first but can be surprisingly deep, as such results preclude in advance all future attempts to establish existence of that object, no matter how much effort or ingenuity is poured into this task. One sees this in a typical instance of the above schoolyard game; one player tries furiously to cleverly construct some impressively huge number {N}, but no matter how much effort is expended in doing so, the player is defeated by the simple response “… plus one!” (unless, of course, {N} is infinite, ill-defined, or otherwise not a natural number).

It is not only individual objects (such as natural numbers) which can be self-defeating; structures (such as orderings or enumerations) can also be self-defeating. (In modern set theory, one considers structures to themselves be a kind of object, and so the distinction between the two concepts is often blurred.) Here is one example (related to, but subtly different from, the previous one):

Proposition 2 (The natural numbers cannot be finitely enumerated) The natural numbers {{\Bbb N} = \{0,1,2,3,\ldots\}} cannot be written as {\{ a_1,\ldots,a_n\}} for any finite collection {a_1,\ldots,a_n} of natural numbers.

Proof: Suppose for contradiction that such an enumeration {{\Bbb N} = \{a_1,\ldots,a_n\}} existed. Then consider the number {a_1+\ldots+a_n+1}; this is a natural number, but is larger than (and hence not equal to) any of the natural numbers {a_1,\ldots,a_n}, contradicting the hypothesis that {{\Bbb N}} is enumerated by {a_1,\ldots,a_n}. \Box

Here it is the enumeration which is self-defeating, rather than any individual natural number such as {a_1} or {a_n}. (For this post, we allow enumerations to contain repetitions.)

The above argument may seem trivial, but a slight modification of it already gives an important result, namely Euclid’s theorem:

Proposition 3 (The primes cannot be finitely enumerated) The prime numbers {{\mathcal P} = \{2,3,5,7,\ldots\}} cannot be written as {\{p_1,\ldots,p_n\}} for any finite collection of prime numbers.

Proof: Suppose for contradiction that such an enumeration {{\mathcal P} = \{p_1,\ldots,p_n\}} existed. Then consider the natural number {p_1 \times \ldots \times p_n + 1}; this is a natural number larger than {1} which is not divisible by any of the primes {p_1,\ldots,p_n}. But, by the fundamental theorem of arithmetic (or by the method of Infinite descent, which is another classic application of reductio ad absurdum), every natural number larger than {1} must be divisible by some prime, contradicting the hypothesis that {{\mathcal P}} is enumerated by {p_1,\ldots,p_n}. \Box

Remark 1 Continuing the number-theoretic theme, the “dueling conspiracies” arguments in a previous blog post can also be viewed as an instance of this type of “no-self-defeating-object”; for instance, a zero of the Riemann zeta function at {1+it} implies that the primes anti-correlate almost completely with the multiplicative function {n^{it}}, which is self-defeating because it also implies complete anti-correlation with {n^{-it}}, and the two are incompatible. Thus we see that the prime number theorem (a much stronger version of Proposition 3) also emerges from this type of argument.

In this post I would like to collect several other well-known examples of this type of “no self-defeating object” argument. Each of these is well studied, and probably quite familiar to many of you, but I feel that by collecting them all in one place, the commonality of theme between these arguments becomes more apparent. (For instance, as we shall see, many well-known “paradoxes” in logic and philosophy can be interpreted mathematically as a rigorous “no self-defeating object” argument.)

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