## Author Archive

## What is Wrong with Cantor’s Diagonal Argument?

Cantor gave two purported proofs for the claim that the cardinality of the set of real numbers is greater than that of the set of natural numbers. According to a popular reconstruction of the more widely known of these proofs, his diagonal argument, we randomly tabulate the real numbers in the interval [0, 1) in an array. I will use a binary version of the table for ease of exposition and give Table 1 as an example:

1 0.**1**0111011000 …

2 0.1**1**010111000 …

3 0.10**1**00101101 …

4 0.011**0**1110001 …

5 0.1000**0**100011 …

6 0.11000**1**10100 …

7 0.100100**0**1110 …

8 0.0010111**0**001 …

9 0.01111011**1**00 …

10 0.001110010**1**1 …

11 0.0101000101**0** …

** .**

** .**

** .**

Table 1

Cantor would ask us to assume for *reductio* purposes that this table is “complete” in the sense that it lists every real number in the interval [0, 1). The items in this list are enumerated by the natural numbers shown in the column on the left-hand side of the table. The digits on the diagonal of the list (boldfaced) give us the infinitely long number *d*:

*d* = 0.11100100110 …

Since *d* is a real number in the interval [0, 1) it must be contained in the list. Let us suppose it is on line 3. We then construct a new number *i* by substituting every 0 after the decimal point in *d* by 1, and every 1 by 0:

*i* = 0.00011011001 …

Cantor asserts that *i* is a real number in [0, 1) but it cannot be found in the putatively complete list, because *i* is guaranteed to differ from every number in the list, since every number’s digit that falls on the diagonal is changed in the corresponding digit of *i*. For example, *i *differs from the number on the 7th line in at least in its 7th digit: the 7th digit of that number, viz. 0, which is also the 7th digit of the diagonal, is turned into 1 in the 7th digit of *i*. Similarly for each and every one of the other numbers in the list. So *i* cannot be a member of the list, even though we had assumed initially that the list was complete.

No list of real numbers in [0, 1) to be set up in a similar table (but with a different ordering of reals) will manage to include all of the real numbers in [0, 1), for we can always construct some number *i* for any given list, which will be left out in the list. It follows, according to Cantor, that no such list will have the items in it being enumerable by natural numbers. Hence, there can be no one-to-one correspondence between the set of natural numbers and the set of real numbers in [0, 1). His conclusion is that the size, or cardinality, of the set of reals even only in the interval [0, 1) is greater than the cardinality of the whole set of naturals. This suffices to establish that the entire set of real numbers are also nondenumerably infinite.

There is a problem with Cantor’s argument above. First, let me call two binary numbers in [0, 1) “inverses” of each other if one number has 1 in a certain location after the decimal point, the other number has 0 in the corresponding location; and if it has 0 in that location, the other number has 1. Thus the numbers on lines 4 and 7 in Table 1, for example, are inverses of each other. So are the numbers on lines 6 and 10. Crucially, *d *and *i* are also inverses of each other. If we are to begin by assuming this list is *complete*, it is natural to require that *every number in the list must have its inverse also included in the list*—otherwise we would have a quick objection to the completeness of the list. Cantor says *i* is not included in the list. But how come? The number *i*’s inverse, namely *d*, *is* in the list in Table 2—we assumed it is on line 3. Since its inverse is in the list, *i* must also be somewhere in the list, *like every other pair of inverses*. But if *i* cannot be in the list, as Cantor insists, then this should be indicative of something suspicious with his entire procedure.

We said that not only *d* but also its inverse *i* has to be in the list, in order for the initial assumption (for *reductio*) of the completeness of the list to be admissible. What happens if *i* *is* in the list? Suppose the inverse *i* of the diagonal is on line 8 in Table 2:

1 0.**1**0111011000 …

2 0.1**1**010111000 …

3 0.11**1**0010 !110 … = *d*

4 0.011**0**1110001 …

5 0.1000**0**100011 …

6 0.11000**1**10100 …

7 0.100100**0**1110 …

8 0.0001101**!** 001 … = *i*

9 0.01111011**1**00 …

10 0.001110010**1**1 …

11 0.0101000101**0** …

** .**

** .**

** .**

Table 2

Because *d* and *i* are inverses of each other, if the diagonal’s 8th digit (in bold red font) is 1, *i*’s 8th digit has to be 0, and if the diagonal’s 8th digit is 0, *i*’s 8th digit has to be 1. Since the diagonal’s and *i*’s 8th digits coincide, therefore, their shared 8th digit can be neither 0 nor 1. Hence inclusion of *i* in Cantor’s table leads to contradiction in the 8th digit of the 8th line. This entails that the 8th digit of *d* (in red font) also harbors contradiction. So when *i* is included in the table, *d*, and crucially *i*, cease to be numbers—because they both have one contradictory digit—and this circumstance derails Cantor’s ploy of inverting the diagonal.

Let me recast the flaw with the diagonal argument as follows:

(1) Assume that we have a complete table of real numbers in the interval [0, 1).

(2) Every real number in the said interval and its inverse have to be included in the table.

(3) *d* is a real number in the said interval.

(4) Therefore *d* has to be included in the table. [1]

(5) Since *d* is in the table, its inverse *i* also has to be in the table.

(6) But when both *d* and *i* are included in the table, they are no longer numbers, as each now contains a contradictory digit. (All of this, of course, spoils Cantor’s strategy of inverting the diagonal.)

(7) Therefore, a putatively complete list of reals in [0, 1) is impossible or paradoxical—*even as an initial assumption* for a Cantorian *reductio* argument.

A Cantorian might still think that I have not refuted what Cantor proved but simply found an alternative way of proving his result that *i* cannot be contained in the list. I tend to think instead that Cantor came up with the correct result with a faulty reasoning or invalid *reductio*. His faulty reasoning was to think that he could assume initially that the table was complete, apparently being oblivious to the complication stemming from such an assumption’s requirement that *d* and its inverse *i* must be included in the table, namely, the clash of *i* and the diagonal at one digit (as shown in Table 2), which turns that digit of the diagonal into a locus of contradiction. Taking inverse of such a diagonal is of course pointless. His constructing *i* would not have looked so straightforward and unproblematic, if he had taken cognizance of this complication brought about by the completeness assumption. My and Cantor’s reasons for why *i* cannot be contained in the table of reals in [0, 1) are not the same. The difference between my position and Cantor’s may sound subtle but is nevertheless real. After all, my reasoning (1)-(7) for the conclusion that *i* cannot be contained in the table is totally different from Cantor’s. [2] Cantor thinks, (a) there is no problem with the diagonal of the table, so he can perform inversion on it, and (b) *i is* a real number (although one that is not in the table). I claim that, because of the initial assumption of completeness, (a’) the diagonal of the table becomes unsuited for inversion (recall the ‘**!**’ sign in Table 2 on the path of the diagonal), and (b’) *i* and *d *become non-numbers.

But why insist on listing the reals in [0, 1) in a table? Cantor does so and this is where the diagonal argument’s big fallacy lies. The problem is not that reals are nondenumerable as Cantor thought; the problem is with listing reals in the form of a table. Such a table appears to be a natural and innocuous proposal to display the totality of real numbers, but this appearance is deceptive. In a supposedly exhaustive table of reals there would have to be an element—the diagonal—that must cross each of the numbers in the table, including the diagonal number *d* and its inverse *i*, at some digit, and this gives rise to the problems we have pointed out. There is an alternative way of displaying the totality of real numbers—via binary trees—which raises no problems or contradictions, since there is no diagonal on a tree. Moreover, as I show elsewhere, the reals (not only those in the interval [0, 1) but the entire set of real numbers) *can* be enumerated by natural numbers using this alternative method. [3]

__________________________________________

[1] If *d* is not included in the table, even though we know that it satisfies the condition of being a real number in [0, 1), then Cantor wouldn’t need to bother to find a number *i* which is not in the table—*d* would fill the bill. And constructing *i* would be superfluous.

[2] Notice that (3) and (6) contradict, hence my (1)-(7) has the structure of a *reductio* argument. Although my initial assumption (1) is the same as Cantor’s initial assumption, my *reductio* is clearly different from Cantor’s. Indeed, my (6) is in conflict with Cantor’s strategy.

[3] My much more detailed criticisms of Cantor’s arguments and my way of showing that reals cannot have a higher cardinality than naturals can be found at:

https://www.academia.edu/37229455/CONTRA_CANTOR_HOW_TO_COUNT_THE_UNCOUNTABLY_INFINITE_

## I have a dream! But I can’t remember it…

In my previous post, “Is Truth Beneficial and/or Socially Constructed?,” I mentioned as a counterexample to the pragmatist theory of truth a nightmare a person had which she did not tell anyone about and kept as a secret for the rest of her life. The nightmare was so horrible and embarrassing that every time she remembered her nightmare, she was disturbed. Her life became a nightmare of sorts because of that nightmare.

Actually this kind of scenario is very rare in real life. The fact is that we tend to forget our dreams and nightmares soon after waking up. Even before we get up from bed, most of the content of our dream has already evaporated from our memory. We remember only very few, if any, of our dreams and nightmares in the rest of our lives. The ones we remember for a while are the ones which were extremely interesting or shocking for us, or those we had the chance to tell other people about on many occasions, which kept our memory of them alive. Ask yourself how many of your dreams and nightmares you still remember. I bet very few, if any.

The interesting thing is that we forget even the most vivid of our dreams and most frightful of our nightmares in the twinkling of an eye (unless our memory of them is reinforced by telling other people about them or by intentional recalling, for example). We forget our dreams even though some of them are *more vibrant* than certain waking experiences which we remember for much longer time.

Psychologists and brain physiologists tell us that dreams serve a useful function for our brain. So we have to have them. But it seems we also have to forget them fast after having them. I think there is a simple evolutionary explanation of this phenomenon. If we were to remember our dreams long after we woke up, we would be disposed to confuse the memories of our dreams with the memories of our waking experiences. Suppose I have a dream in which a friend of mine does something evil to me or an enemy of mine does a big favor for me. If my brain were to retain as lively a memory of that dream as the memories of my real life experiences, I might mistakenly think the contents of my dream correspond to some real experiences of mine that occurred in the past, and my attitude towards my friend or towards my enemy would unnecessarily be affected by that mistake. Such disorientations clearly would have negative survival value and therefore would be blocked by the mechanisms of human evolution. Hence the elusiveness of our dream contents.*

## Is truth beneficial and/or socially constructed?

There are several varieties of the pragmatist theory of truth. Since, according to the pragmatist theories, what we take to be truth is dependent on our pragmatic interests, rather than being “representations of reality,” the pragmatist theories can be regarded as anti-realist theories of truth. According to C.S. Peirce, who is one of the important figures in the pragmatic tradition, truth is, briefly, beliefs socially agreed upon in the long run. Hence “reality” is something socially constructed and is based on consensus. Another important figure, William James, thought that truth is something that has instrumental value. For James “facts” are our mental constructs which prove beneficial in the long run. The popular versions of especially the instrumentalist variety of pragmatism can be found in slogans like, “The truth is what works,” “Truth is what is convenient to believe,” “A proposition is true if believing it has advantageous results.” For the purposes of what follows, I will take the pragmatist theory to be claiming the following:

(PT) Proposition S is true IFF believing that S yields beneficial results in the long run.

I assume that (PT) is shared fully or partly by all pragmatist theories. (One could substitute “has pragmatic value” for “yields beneficial results” in (PT) to stay closer to the letter of the title “pragmatic theory of truth.”)

First off, a counterexample. Suppose I had a terrible nightmare. Every time I remember it, I get the creeps.* I don’t tell anyone about it, because my nightmare is also kind of embarrassing and I am afraid people will make fun of me or will insist that I go see a shrink to get it analyzed—which I’d hate to do. So I keep silent about it for the rest of my life. Thus my belief that I had that nightmare produces no ostensible benefits whatsoever in my life—if anything, every time my belief is enlivened by my recollection of the nightmare, this does nothing but disturb me. As I tell no one about it, my belief yields no useful results for anyone else either. It might even make me edgy in my dealings with some other people at least for a while, and this is not going to be beneficial for any of the parties. So, no useful outcome ensues from my belief either for myself or for any portion of humanity. I have no idea what caused my nightmare, so I don’t have a clue how I can prevent my or someone else’s having a similar nightmare in the future. And since I refuse to consult a shrink about it, she is not getting any monetary or academic benefits out of it either.

Yet, it is *true* that I had the nightmare, even though no one reaps any benefits out of my belief that it happened.

## A more devastating version of the Raven Paradox

C.G. Hempel’s “Raven Paradox” involves derivation of the intuitively unpalatable conclusion that observation of things like a white shoe or a rainbow confirms the raven hypothesis, “All ravens are black.” Here’s how it goes. An earlier author Jean Nicod had put forward the following criteria for confirmation of hypotheses of the form “All A’s are B’s”:

Observation of an object which has the property of being an A and also the property of being a B *confirms* “All A’s are B’s.”

Observation of an object which has the property of being an A but not the property of being a B *disconfirms* “All A’s are B’s.”

Observation of an object which does not have the property of being an A neither *confirms nor disconfirms* “All A’s are B’s.”

Add to these criteria the following highly plausible claim, which Hempel called “the equivalence condition”:

If an hypothesis H1 is logically equivalent to another hypothesis H2, then, if an observation O confirms H1, then O also confirms H2.

The equivalence condition sounds perfectly true, because to say that H1 and H2 are logically equivalent is to say that H1 and H2 make exactly the same claims about the world. Thus if a piece of evidence confirms one of the hypotheses, it must equally confirm the other one.