Gustav Fechner
Fechner, along with Wilhelm Wundt and Hermann von Helmholtz, is recognized as one of the founders of modern experimental psychology. His clearest contribution was the demonstration that because the mind was susceptible to measurement and mathematical treatment, psychology had the potential to become a quantified science. Theorists such as Immanuel Kant had long stated that this was impossible, and that therefore, a science of psychology was also impossible.
Introspection
It has often been claimed that Wilhelm Wundt, the father of modern psychology, was the first to adopt introspection to experimental psychology though the methodological idea had been presented long before, as by 18th century German philosopher-psychologists such as Alexander Gottlieb Baumgarten or Johann Nicolaus Tetens. Also, Wundt's views on introspection must be approached with great care. Wundt was influenced by notable physiologists, such as Gustav Fechner, who used a kind of controlled introspection as a means to study human sensory organs. Building upon the pre-existing use of introspection in physiology, Wundt believed the method of introspection was the ability to observe an experience, not just the logical reflection or speculations which some others interpreted his meaning to be. Wundt imposed exacting control over the use of introspection in his experimental laboratory at the University of Leipzig, making it possible for other scientists to replicate his experiments elsewhere, a development that proved essential to the development of psychology as a modern, peer-reviewed scientific discipline. Such exact purism was typical of Wundt and he instructed all introspection observations be performed under these same instructions "1) the Observer must, if possible, be in a position to determine beforehand the entrance of the process to be observed. 2) the introspectionist must, as far as possible, grasp the phenomenon in a state of strained attention and follow its course. 3) Every observation must, in order to make certain, be capable of being repeated several times under the same conditions and 4) the conditions under which the phenomenon appears must be found out by the variation of the attendant circumstances and when this was done the various coherent experiments must be varied according to a plan partly by eliminating certain stimuli and partly by grading their strength and quality".