How Many Impossible Images Did Escher Produce?
Chris Mortensen, Steve Leishman, Paul Quigley, Theresa Helke · The British Journal of Aesthetics · 2013
In this article we address the question of how many impossible images Escher produced. To answer requires us first to clarify a range of concepts, including content, ambiguity, illusion, and impossibility. We then consider, and reject, several candidates for impossibility before settling on an answer.