Final answer:
An assumption required for the art historian to draw his conclusion properly is that art used to be more shocking than it currently is.
Step-by-step explanation:
An assumption that is required for the art historian to draw his conclusion properly is that art used to be more shocking than it currently is. The conclusion drawn by the art historian likely revolves around the historical evolution of the shock value in art. For this conclusion to be valid, the assumption that art used to be more shocking in the past is necessary. It implies a comparison between the level of shock in historical art and contemporary art.