 Although a number of early modern philosophers had aimed to initiate the scientific study of the mind, it wasn’t until the middle of the 19th century—with the appearance of quantitative introspective methods, especially regarding sensory consciousness—that the study of the mind took shape as a progressive, mathematical, laboratory-based science. Early quantitative psychologists such as Helmholtz (1856/1962), Fechner (1860 [1964]), and Wundt (1896 [1902]) sought quantitative answers to questions like: By how much must two physical stimuli differ for the experiences of them to differ noticeably? How weak a stimulus can still be consciously perceived? What is the mathematical relationship between stimulus intensity and the intensity of the resulting sensation? (The Weber-Fechner law holds that the relationship is logarithmic.) Along what dimensions, exactly, can sense experience vary? (The “color solid” [see the link to the Munsell solid in Other Internet Resources, below], for example, characterizes color experience by appeal to just three dimensions of variation: hue, saturation, and lightness or brightness.) Although from very early on, psychologists also employed non-introspective methods (e.g., performance on memory tests, reaction times), most early characterizations of the field stood introspection at the center. James, for example, wrote that “introspective observation is what we have to rely on first and foremost and always” (1890 [1981, 185]).