Suppose you are accosted by an omniscient highwayman – and instead of demanding your money or your life, he is after something altogether more tricky: empirical truth. Tell him something empirically true and you escape unharmed. What should you say*?
Consider, for a moment, externalist definitions of knowledge – for example that a belief is the production of a reliable mechanism for producing truth. Would this knowledge, if I had it, be of any use to me here? It would not, as it is not a condition of externalist knowledge that I can internally discern what is true and what is not true. My perceptual beliefs may be reliable, and if they are reliable then I know them, according to the externalist. But that knowledge is of no comfort to me in this situation, as it doesn’t help me make a choice whether to assert one proposition or the other. P may be the production of a reliable mechanism and Q may not be, but if I have no idea whether P or Q are produced by reliable mechanisms, I have no reason consciously available to me to assert P or Q to the highwayman.
If someone demands of me that I speak truth, in order to be able to make choices based on reason in order to meet that demand, it must be the case that any reason for believing something be available to me. Those are the only kinds of reasons that are any good to me if I’m to actively try and stay alive in this situation. I need to consider my internal justifications for believing any particular proposition to be true.
Now, suppose I’m considering whether proposition P would be a good candidate to assert to the highwayman. I look at my reasons for P. Say I believe P because of [R → P] and R. But why then not assert either R or [R → P]? Surely P cannot be on a better internal epistemic footing than both of those two propositions put together? For if my belief that P is truly justified only by [R → P] and R, then P would have the same evidential status as [[R → P] & R]. Since you cannot increase the probability of a proposition being true through conjunction, I should be better off asserting either R or [R → P].
Suppose then I choose to assert R. Why do I believe R? If I give reasons, then I am better off asserting the reasons for R than R itself. I can never get to a point where I can rationally assert one proposition rather than any other. This is true even if I gave some coherentist-style justification for a proposition, as the issue with coherentism is there is absolutely zero reason to believe than a coherent set of propositions remotely resembles the truth.
Now, fortunately we are not usually in the position of having to assert truth to an omniscient psychopath. But consider the following not-implausible epistemic norm:
A) You should assert only what is justified
As our highwayman example shows, if we go about actually trying to meet such a test based on what is consciously available to us, we’ll never get to a point of justification. External justification is of no help to us when consciously trying to meet epistemic norms. And that, in my view, is the real challenge of the sceptic. There is nothing intrinsically wrong with a belief having the property of being produced by a reliable mechanism, or of its being caused by the truth of its content. It’s a genuine property that any number of our beliefs could have, but since the justification is external it doesn’t help us in the deliberate practice of choosing what we should believe or assert. If we were to instead invoke a stronger epistemic norm:
B) You should only assert what you
know can discern to be true
And combine this with the ‘ought-implies-can’ principle, the fact that we cannot give any internal reasons for believing one proposition rather than another from our own internal perspective leads to the unhappy conclusion that
C) One ought not to assert anything.
*Some clever-clogs is going to figure out that the optimal strategy in this situation would be to say ‘My life is being threatened’. If it’s true then you pass the test, if it’s not then your life isn’t threatened anyway. So let’s also assume than our omniscient highwayman also has an irrational dislike for clever-clogs