The fundamental visual property that drives 3D stereoscopic technology is the compelling qualitative experience of tangible solid objects, immersive space and realism that is lacking in conventional 2D displays. This qualitative visual phenomenon, referred to as ‘stereopsis’, is widely assumed to be a by-product of binocular vision. However, its underlying cause, variation and functional role remain largely unexplained. In this theoretical paper I briefly present an alternative hypothesis that stereopsis is not a phenomenon restricted to binocular vision, but a more basic qualitative visual property related to the perception of egocentric distance and scale. I review recent empirical evidence showing that stereopsis is not simply a product of binocular disparities or the mere perception of “more depth”. The theory and results imply critical distinctions between the qualitative experience of stereopsis and the quantitative perception of 3D structure. I describe how this alternative view has important implications for the perception of scale and realism in both conventional and stereoscopic display systems; e.g., perception of miniaturization (puppet-theater effect) and gigantism. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE).