The reafference theory faces a number of objections. First, the theory is an extension of von Holst and Mittelstädt’s reafference principle, according to which efference copy is used to cancel out shifts of the retinal image caused by saccadic eye movements. The latter was specifically intended to explain why we do not experience object displacement in the world whenever we change the direction of gaze. There is nothing, at first blush, however, that is analogous to the putative need for “cancellation” or “discounting” of the retinal image in the case of prism adaptation. As Welch puts it, “There is no visual position constancy here, so why should a model originally devised to explain this constancy be appropriate?” (1978: 16).