If someone claims not to, then in broad terms ask him if he would like his assertion to be built into the rules of inference used when others respond to what he has to say. If he truly believes that the argument is valid, then he will have nothing to fear, will he? If not, perhaps his fear will make an honest man out of him.

While this approach can be an effective way of reducing the impact of dishonest posturing on the formation of consensus by discussion, would it be a harsh way of treating someone who is merely being sincerely foolish? If applied without the use of common sense, it would be. If he is hiding from an axe murderer, I would not say that one should decline him sanctuary just to call his bluff or teach him the error of his ways, as the case may be (*). That would be wrong. However, one should not shy away from the opportunity to engage in a little gentle teasing when the situation arises, and his insincerity on this point is revealed. By learning not to take this attitude, he will make himself far easier to be around, a blessing for him and those around him alike.

Question : So he lied a little. Isn't this a harsh response
..................... to an innocent little bit of foolishness?

... Answer : No, this is a mild response.

Let us consider the broader principle being promoted here. Namely, that one can't legitimately perceive anything until others perceive it with one. On those terms, what happens to independent thought? What happens to the ability of the individual to opt out of a popular consensus that has left reason and reality behind, once he's given others the power to keep him from seeing anything that would contradict that consensus? There is nothing "innocent" about this. It's one of the conceptual building blocks of totalitarianism. To promote this for the sake of a little attitude, gives selfishness a whole new reprehensible dimension.

Let's continue.

(*) An extreme illustration, but on point: Suppose our hypothetical contrarian who insists that knowledge must be collective, never personal, finds himself pursued by somebody with an axe. By his logic, if nobody else has seen the maniac he is running from wield an axe, he would have to conclude that he has no reason to believe that the axe wielding maniac will do him harm and no reason to flee him. Or seek shelter to get away from him.

A nice classroom exercise, that argument, but we're not suggesting that you take your zeal for philosophical consistency that far.