What crucial assumption underpins the argument from dreaming and indeed the subsequent argument concerning the possibility of an evil spirit who is radically deceiving him?
I am not sure just what you have in mind by those two arguments. But if you mean that argument that since, for all we know, we might be dreaming, or that for all we know, we might be being deceived an Evil Demon, that none of our beliefs justified by sense-perception that we know anything are adequately justified, so that any belief we have that is justified by sense-perception can be known. The upshot is that if we believe that we know anything justified by sense-perception, that belief that we know anything based on sense-perception is false. There are several crucial assumptions here, but one of them is that any belief that we know must be adequately. justified. Another is that no belief that we know that is based on . sense-perception is adequately justified. Still another crucial assumption is that unless our justification for a belief is such that it guarantees
the truth of the belief, such justification is inadequate. And this entails that only deductive justification is adequate justification. I don't know what crucial assumption your teacher has in mind, but you would know if you attended and listened in class.