Monday, February 23, 2026

A variant of the preface paradox

Imagine being given a list generated as follows, without being told how it was generated:

  1. A randomly chosen set of 2000 mutually consistent propositions that you justifiedly believe, subject to the constraint that they are consistent.

Given your fallibility, it’s highly probable that the list contains multiple falsehoods. As you look over the list, you say to yourself about each one: “Yup, that sounds right.” However, given what you know about your fallibility, you should also say: “I think some of these are false, though.” You will thus incline to disbelieve the conjunction of these propositions.

But suppose instead you were handed a list generated as follows, again without being told how it was generated:

  1. A randomly chosen set of 2000 propositions that you know.

This list would look to you just like the first one. Thus, you would say about it exactly what you would have said about the first one. To each one, you would say: “Yup, that sounds right.” But then on reflection you would say: “I think some of these are false, though.” You will thus incline to disbelieve the conjunction of these propositions, just as in the first case. However, while in the first case you are correct in disbelieving, in the second you are mistaken in disbelieving.

But in any case, if you incline to disbelieve something, then you don’t know it. Hence, you do not know the conjunction of the 2000 randomly chosen propositions that you know, and so knowledge is not closed under conjunction.

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