The Müller-Lyer illusion is an optical illusion devised by German sociologist Franz Carl Müller-Lyer in 1889, where viewers misjudge the length of arrow shafts based on whether fins point inward or outward. The effect occurs universally across all human cultures and even affects those who have been blind, suggesting it is neurologically constant rather than culturally determined. Proposed explanations include Richard Gregory's depth-cue hypothesis involving size constancy, challenged by alternative theories emphasizing whole-part determination and space-time reciprocity.