In Greg’s comment here, he notes correctly that when we form beliefs, we often do so on the basis of things assumed. In the example there, my wife says something, and I respond in a way that shows that I assumed that she wasn’t trying to trick me and that she thought there was something surprising about the case.
My question here is just what an assumption is. I agree that I assumed these things in the exchange in question, but it is not true that I thought of these claims, or drew inferences running through the claims either.
One might think, then, that an assumption is a non-occurrent mental state akin to dispositional beliefs. That will imply, of course, that you can be in non-occurrent mental states like this without having first been in the relevant occurrent state. Pollock denies this for beliefs, and the best way to deny it is to distinguish between dispositional beliefs (which require prior occurrent beliefs with the same content) and dispositions to believe (which do not require prior occurrent beliefs). If we don’t want to try to settle that dispute, we might say an assumption is either a dispositional belief or a disposition to believe.
This view may be correct, but I’m not sure. In many cases where our assumptions are pointed out to us, we experience chagrin at the realization. Upon thinking about the particular propositional content in question, we do not embrace it. Instead, we reject it.
This point raises a problem for conditional analyses of dispositions, and may be avoided by what C.B. Martin dubs “realism” about dispositions. In such a case, the very conditions for the triggering of the disposition cause it to be lost.
Even though the view that assumptions are dispositions can be retained in this way, I’m not sure we have an explanation of the chagrin in question. To experience chagrin in such an immediate fashion, it would seem that we need some disposition in place that runs contrary to the assumption in question. Suppose, for example, that you are strongly averse to racism, but respond on a given occasion in terms that you agree are accurately described in terms of assuming that people of a particular race are more dangerous. You experience chagrin upon having this fact pointed out to you.
Your chagrin depends upon your aversion to racism, and such aversion involves, I would expect, cognitive commitments. You believe, or are committed to, lots of claims, including the exact opposite of the assumption underlying your response. But in general we don’t want to try to make sense of ascribing both the belief that p and the belief that not-p to an individual (except in cases where modes of presentation explain away the absurdity in question). Nor do we want to say, I think, that some of your beliefs about the races somehow went out of existence during the period of your assumption. Nor will appeal to degree or strength of belief help, so long as we are still willing to countenance the reality of the distinction between beliefs and non-beliefs (even if the threshold varies by context).
So maybe we must think of assumptions as dispositional mental states, or dispositions toward mental states, that are not reducible to beliefs?