It is sometimes said that there has been a "paradigm shift" in the field of assessment over the last two or three decades: a new preoccupation with what learners can do, what they know or what they have achieved. It is suggested in this article that this change has precipitated a need to distinguish two conceptually and logically distinct methodological approaches to assessment that have hitherto gone unacknowledged. The upshot, it is argued, is that there appears to be a fundamental confusion at the heart of current policy, a confusion occasioned by the demand to know what learners know and compounded by a failure to recognise what this properly entails for assessment methodology. (Contains 4 notes.)