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The Platonist Rod Paradox

The formulation of this paradox was inspired by the Platonist interpretations of the Balls in the Urn paradox. (Footnote: Unlike the case of Balls in the Urn paradox, in this paradox no piece that is added is ever subsequently removed.)


Consider the following:


We start with a rod of exactly 1 unit length in an infinitely large container


At 60 seconds to noon 9 rods of length 1 are added, and then 9/10ths is cut off each rod, and the remaining pieces are set end to end, so the total length is 1 unit, as before.


At 30 seconds to noon 99 rods of length 1 are added, and then 99/10ths is cut off each rod, and the remaining pieces are set end to end, so the total length is 1 unit, as before.


At 15 seconds to noon 999 rods of length 1 are added, and then 999/10ths is cut off each rod, and the remaining pieces are set end to end, so the total length is 1 unit, as before.


And so on…


Each step takes zero time to accomplish.


Now consider the following questions:

  1. At noon what is the length of the composite rod?
  2. At noon how many pieces are there?
  3. At noon what is the length of each piece?


A typical response is that:

  1. The length of the composite rod is 1 (since its length is always 1 after each step).
  2. There are infinitely many pieces (since their number increases at every step and there is no limit to the number of steps).
  3. The length of each piece is zero (since the size decreases at every step and there is no limit to the number of steps).


But this is not the result that the Nested Intervals Theorem of set theory gives, (Footnote: For example, as described in Ethan D Bloch’s book, The real numbers and real analysis, Springer Science & Business Media, 2011, Bloch: The real numbers and real analysis: Details.) and which states that:

  1. the intersection of infinitely many nested intervals is not empty, and
  2. if the limiting value of the left-hand ends of the intervals and the limiting value of the left-hand ends of the intervals are identical, then the intersection is a single point which is that limiting value.


But the limiting value of all the left-hand ends are all zero, and the limiting value of all the right-hand ends are also all zero, which gives the result that at noon the rod is a single point rather than a rod of length 1. This is in direct disagreement with (1) and (2) above, hence the paradox.


So how does the paradox arise?


It arises because the process never actually completes. As in the case of the Balls in the Urn paradox, the reference to time in the description is a red herring, see the section Time as a red herring on the page Balls in the Urn paradox. Since the process cannot complete, the pieces never actually become of zero length, and must always have some finite length, regardless of how small that might be. And there is no step where the left-hand end of any piece becomes coincident with the left-hand end of another piece.


So the correct answer to (ii) is that there is no limiting value to the quantity of pieces, noting that there is no step at which the value actually becomes infinite (note that the term “infinite” is frequently treated as though it refers to a particular quantity, whereas in fact, it refers to a situation where there is no upper limit to a quantity).


And the correct answer to (iii) is that the limiting value of the length of the pieces is zero. Since the process never terminates, the pieces must always have some length greater than zero, regardless of how small that length can be.


In this way, the apparent paradox is resolved. The Platonist notion that an infinitely recursive process can actually “complete” leads to the paradox. This goes hand-in-hand with the Platonist notion that mathematical things actually exist independently of any definition. According to Platonist beliefs, every mathematical entity (in the above definition, every interval that can be defined by that definition) already somehow “exists” prior to the creation of the definition, and there is a non-physical mathematical “reality” where every step in the process already “exists” prior to the creation of the definition.

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Diverse opinions and criticisms are welcome, but messages that are frivolous, irrelevant or devoid of logical basis will be blocked. Difficulties in understanding the site content are usually best addressed by contacting me by e-mail. Note: you will be asked to provide an e-mail address - any address will do, it does not require verification. Your e-mail will only be used to notify you of replies to your comments - it will never be used for any other purpose and will not be displayed. If you cannot see any comments below, see Why isn’t the comment box loading?.

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The Lighter Side


Paper on the diagonal proof

There is now a paper that deals with the matter of language and the diagonal proof, see On Considerations of Language in the Diagonal Proof.

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Other recently added pages

The Myths of Platonism


Goodman’s Paradox


The Platonist Rod paradox


The Balls in the Urn Paradox


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Lebesgue Measure

There is now a new page on a contradiction in Lebesgue measure theory.

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Easy Footnotes

I found that making, adding or deleting footnotes in the traditional manner proved to be a major pain. So I developed a different system for footnotes which makes inserting or changing footnotes a doddle. You can check it out at Easy Footnotes for Web Pages (Accessibility friendly).

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O’Connor’s “computer checked” proof

I have now added a new section to my paper on Russell O’Connor’s claim of a computer verified incompleteness proof. This shows that the flaw in the proof arises from a reliance on definitions that include unacceptable assumptions - assumptions that are not actually checked by the computer code. See also the new page Representability.

Previous Blog Posts

Moderate Platonism

Descartes’ Platonism

The duplicity of Mark Chu-Carroll

A John Searle Inanity

Man versus Machine

Fake News and Fake Mathematics

Ned Block’s Blockhead

Are we alone in the Universe?

Good Math, Bad Math?

Bishops Dancing with Pixies?

Artificial Intelligence

Cranks and Crackpots

The Chinese Room


For convenience, there are now two pages on this site with links to various material relating to Gödel and the Incompleteness Theorem


– a page with general links:

Gödel Links


– and a page relating specifically to the Gödel mind-machine debate:

Gödel, Minds, and Machines

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