According to legend, on this date in 1850, Gustav Theodor Fechner arose from his sleep armed with wholly new method to attack the problem studying the mind. Rather than relying on introspective reports of what was going on in people’s minds, scientists could, instead, vary the intensity of some external physical stimulus and ask the “participant” (as we now call them) whether s/he could detect any difference perceptually. For instance: “Does this weight seem heavier than that one?” “Does this light seem brighter or greener than that one?” “Does sound seem louder or higher than that one?”
By using this method rigorously, Fechner was able (borrowing on some earlier work by Ernst Heinrich Weber) to develop a mathematical law putatively mapping changes in physical intensity on to changes in perceived intensity. Called the Weber-Fechner Law, it says that the relationship between the physical and the psychological is logarithmic (i.e., linear changes in perception require proportional changes in the physical stimulus)
The insight was developed by Fechner into the field of psychophysics, and its basic methodology spread throughout the study of mind, culminating in Wundt’s launch of the discipline of experimental psychology (or “physiological psychology,” as he called it) in the mid-1870s.
For more on Fechner, see the Wikipedia article.