This chapter examines the development of a geometrical framework for understanding and explaining spatial aspects of visual perception, including perception of the sizes, shapes, and positions of things in the field of view. The structure of this framework is built on the fact that vision typically occurs in straight lines (rectilinearly). Within this framework, the chapter selectively focuses on size perception. This focus allows for a comparative examination of how a single problem was treated geometrically by various theorists, ancient, medieval, and modern. The theorists examined are Euclid and Ptolemy, who were extramissionists, and Ibn al-Haytham, Kepler, Descartes, and Berkeley, each of whom adopted, in one way or another, an intromissionist scheme. In comparing Descartes and Berkeley, notice is taken of Berkeley’s interpretive bent in treating Descartes’s account of distance perception in a way that requires mental calculation, where Descartes sometimes offered psychophysiological mechanisms (avoiding mental calculation).