There’s a serious aspect to what Stuart jokingly says here. Philip Pettit and Michael Smith put a finger on it in their discussion of what they call “the conversational stance” in “Freedom in Belief and Desire” [pdf].
When we engage in serious conversation about what to believe or do, Pettit and Smith observe, we assume, among other things, that our interlocutor can, and will, change her beliefs (about the way things are) and evaluations (about what to do) in light of evidence. We assume, further, that she will adjust her desires and assess her plan of action in light of these evaluations. (So, they will go on to argue, we hold her responsible as a free thinker and as someone possessed of free will.) Otherwise, there is no point in having the conversation, and there might even be reason to fear that we are involved with a zombie or psychopath:
Were you to think that your interlocutor lacked the dispositions to register and respond to the demands of the norms governing evaluations that you both countenance, and lacked them even in the provisoed measure allowed, you would either have to put his evaluative understanding or commitment in serious question or you would have to regard him as something close to a zombie or a psychopath. How could your interlocutor agree that doing such and such is irrational, so you will ask, but not see that the prescription applies to him? Or, if he does admit it applies to him, how could he fail to adjust his desires and actions accordingly? In particular, how could he fail to do these things, when the failure is not to be explained by reference to familiar obstacles [such as fetishes and obsessions, disabling moods and passions]? The only answer available would seem to be that he is not seriously or sincerely involved in the business of practical evaluation, or that if he is, then he is not reliably attuned to the practical values in question. In either case, you lose solid grounds for authorizing him as a conversational interlocutor. You must cease to see any point in conducting a conversation that is supposed to bear on how he should behave.