I had a thought the other night about how the link between 'ought' and various knowledge ascriptions might involve conversational implicatures rather than entailment relations. I then discovered that Walter Sinnott-Armstrong basically worked out the view I was kicking around. On his view, when we use 'ought' in an advisory context and say 'S ought to X' there's a CI to the effect that S can follow the advice we've just given. I'm reading a paper by Bart Streumer criticizing Sinnott-Armstrong's view. I'm hoping that the work on the 'ought'/'can' relationship might be useful for thinking about the 'ought'/'knows' relationship as I'm trying to reconcile the view that ignorance is an excusing condition rather than, say, an enabling/disabling condition with certain observations about the use of 'ought' in cases where we're forced to come to a judgment knowing we do not know certain seemingly relevant facts about the consequences of pursuing one of the available options (e.g., Prichard's stop sign, Parfit's mine shaft).
An example. Jones' home has been burgled and Jones has been tied to a chair. Smith arrives. Jones does not answer the door because he is tied to the chair. Smith says:
(1) If you've been burgled, you ought to get out of your chair and call the police.
It seems natural for Jones to respond:
(2) I'm tied to the chair, so I cannot call the police.
The response seems perfectly apt. The question is whether it shows (1) to be false or not? Sinnott-Armstrong says it doesn't show (1) to be false. Rather, in asserting (1), one implies that the advice offered can be followed and (2) speaks to that implication. Streumer doesn't buy this response. He asks us to consider two further claims. Smith says:
(3) I know you cannot call the police, but that does not make it any less true that you ought to call the police.
(4) Don't be ridiculous. It cannot be true that I ought to call the police if I cannot do so.
He thinks that Jones' reply is a natural one and that this cannot be so if 'ought' CI's 'can'. He writes, "if 'ought' conversationally implicated 'can', Smith would be cancelling the implicature when he says that he knows that Jones cannot call the police, and it would then not be natural for Jones to reply that Smith's claim cannot be true".
The thing is that I simply don't think (4) is natural. It doesn't strike me as an obviously appropriate thing to say and (3) strikes me as perfectly coherent.
Here's an example that I think suggests that Sinnott-Armstrong is winning thus far. Suppose Lanie is doing some home repair and really dislikes some lamps. I tell her that the ugly lamps ought to be left alone because the typical home buyer likes ugly lamps. It's all but analytic that most people have terrible taste. I say:
(5) You really ought to leave those lamps alone.
She insists on painting them, in which case I'd say:
(6) In that case you ought to paint them silver.
Now, consider two views:
(I) 'Ought' conversationally implies 'Will'
(II) 'Ought' entails 'Will'
A defender of (II) might say that when we know someone will not pursue some course of action, we say that they ought to pursue some other course of action and avoid other options. That, they say, is evidence that 'Ought' entails 'Will'. It's essentially the same evidence we get for 'Ought' entails 'Can' when we observe that upon learning that Jones is tied to a chair we take back the claim 'You ought to call the cops' and say instead 'You ought to have screamed for your neighbors to help untie you'. However, there is no one who thinks that (II) is true and in explaining the naturalness of (6) and taking back (5) will appeal to something along the lines of (I). I think this example shows that Sinnott-Armstrong was right to remind us that the fact that an agent ought to do one thing does not show that they should not do other things. Lanie really ought to leave those lamps alone, but since she will not, she really ought to paint them silver. Jones really ought to call the cops, but the reason the crooks tied him to his chair was to prevent him from doing what he ought to! That does not change the fact that he should have shouted for help since he could not untie himself.