Actually getting on to Keith’s paper, he makes a lot of use of the knowledge rule for assertion – say only what you know. I think this is a rule, but I don’t think it’s the only rule of its type. It certainly isn’t the only conversational rule, but some people, including Tim Williamson, and I think Keith, think it’s a special rule. I think it’s not even a rule as such, it’s rather a consequence of the following general principle.

KEITH’S MOOREAN PRINCIPLE: When S says that p, S represents herself as knowing that p. (Also known as TIM’S PRINCIPLE.)

BRIAN’S PRINCIPLE (Stolen, and possibly distorted, from Frank Jackson): When S says that p, she represents herself as having all things considered reasons to accept that p, and that at least some of these reasons were not previously available to at least some of her audience.

I don’t have an argument for Brian’s Principle, but it seemed right to me at the time I was reading Keith’s paper. Maybe I’ll write something one day arguing for it. In keeping with this blog, I should note that at one stage last night I did start to worry quite seriously about it. It seemed I was doing things that at least sort of counted as making assertions all through the night, even to the taxi driver on the way to the bus station home, but it was far from clear I was constitutionally capable of representing anything quite as complicated as all that. Maybe that’s wrong though – even ten vodka and somethings can only do so much damage to my representational capacity.

If Brian’s Principle is right, some of Keith’s arguments from the context-sensitivity of assertability to the context-sensitivity of knowledge ascriptions won’t go through.

In any case, I think the Jason/John relevant alternatives theory does a better job than contextualism at explaining the variability we do see in assertability conditions, though that’s a story for a longer post.

UPDATE: Note that the principle I call Keith’s Principle is not original to Keith or Tim – it occurs explicitly in Peter Unger’s 1975 book Scepticism and is mentioned in passing in Max Black’s 1952 article “Saying and Believing”. Thanks to Gil Harman for pointing out some of the historical sources here.