Abstract

The paper examines the philosophical implications of a phenomenon in the psychology of perception: the Mueller-Lyer illusion. In this visual effect, the impression is created that a horizontal line enclosed by acute angles is shorter than a similar line flanked by obtuse angles, though the lines are of equal length when measured with a ruler. While the Mueller-Lyer effect may be merely illusory when one adheres to the metrical laws of perceptual geometry based on Euclid, it is suggested that, from a phenomenological standpoint, the effect has a validity of its own. Going further, the Mueller-Lyer configuration is adapted to provide an example of a form of visual experience that specifically violates one of Euclid's basic postulates. The article closes by exploring the possibility of non-Euclidean visual geometries in relation to the concept of causality, Einsteinian relativity, and anomalous phenomena.