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.

1 comment:

Martin Cooke said...

If I am shown a randomly chosen set of 2000 propositions that I know, then I would be inclined to disbelieve the conjunction of those propositions, in the sense of doubting that they were all true, as I look at them all. But I would continue to believe each and every one of those propositions. I would have a simultaneous belief in each and every one of them. So it seems to me that my doubt that they were all true would not get in the way of my knowing the conjunction of those propositions.

And after all, I might wonder if I had read those 2000 propositions perfectly. I might have all sorts of doubts about that set. Doubts of that kind could hardly get in the way of my knowing the conjunction of all the things I know. So how could my doubt that those 2000 propositions were all true get in the way of my knowing the conjunction of those propositions?

Just a thought (I do admire the way you keep on blogging about such interesting matters : - )