Peter Klein was just here and gave a stimulating talk on useful falsehoods. The idea is that sometimes you know things even though the evidential path relies essentially on falsehoods. Here’s one of his examples:
You believe that your secretary told you on Friday that you have an appointment on Monday. As a result, you believe that you have an appointment on Monday, and since your secretary is reliable, trustworthy, etc., and what she told you is true, you know that you have an appointment on Monday. But she didn’t tell you this on Friday; she told it to you on Thursday.
The standard defeasibility account Peter defends does not accommodate this example, so Peter’s goal is to characterize useful falsehoods in order to adjust his undefeated justified true belief account of knowledge.
The account he develops takes it’s cue from a remark by Hilpinen that you can acquire knowledge through falsehoods if your false belief is close enough to the truth. Here’s the full account:
The belief that uf is a useful falsehood to S (for acquiring knowledge) by producing a
doxastically justified belief that h iff:
1. uf is false
2. The belief that uf is doxastically justified for S
3. The belief that uf is essential in the causal production of the belief that h
4. uf propositionally justifies h
5. uf entails a true proposition, t
6. t propositionally justifies h
7. Whatever doxastically justifies the belief that uf for S also propositionally
justifies t for S.
The clause that makes this an elaboration of Hilpinen’s remark is clause 5, that the false belief has to entail a true one, where the notion of entailment is something like the relevance notion of it.
There is an alternative, though, and neither Peter nor I can think of a reason that it won’t work. What matters in the secretary case is that you could have come to the conclusion that you have an appointment on Monday by reasoning through the claim that your secretary told you sometime during the past week that you have an appointment on Monday. This claim is true, and is related to the false belief that you actually reasoned through in two different ways. First, the false belief entails the true proposition in question. Second, the false belief is a reason to believe the true proposition (and the reasons for believing the false belief are equally good reasons for believing this true proposition).
So there are two different clause 5’s that could be proposed. There is the alethic version espoused by Hilpinen and clarified in terms of the notion of relevant entailment by Peter. There is also the epistemic version, according to which what matters for that particular clause is just the idea that your false belief propositionally justifies the true proposition in question.
Suppose we call the first view the alethic presupposition view (since x presupposes y in this sense only if x entails y). And suppose we call the second view the epistemic presupposition view. The question then is this: is there any reason to prefer either view to the other in accounting for cases like the secretary case?