The Berkeleyan approach looks to action in order to explain how depth is “added” to a phenomenally two-dimensional visual field. The spatial ordering of the visual field itself, however, is taken to be immediately given in experience (Hatfield &amp; Epstein 1979; Falkenstein 1994; but see Grush 2007). Starting in the 19th century, a number of theorists, including Johann Steinbuch (1770–1818), Hermann Lotze (1817–1881), Hermann von Helmholtz (1821–1894), Wilhelm Wundt (1832–1920), and Ernst Mach (1838–1916), argued that all abilities for visual spatial localization, including representation of up/down and left/right direction within the two-dimensional visual field, depend on motor factors, in particular, gaze-directing movements of the eye (Hatfield 1990: chaps. 4–5). This idea is the basis of the “local sign” doctrine, which we examine in Section 2.3.