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.
Critical psychology
One of the most important and sophisticated books in the field is the "Grundlegung der Psychologie" ("Foundations of Psychology") by Klaus Holzkamp, who might be considered the theoretical founder of critical psychology. Holzkamp, who had written two books on theory of science and one on sensory perception before publishing the "Grundlegung der Psychologie" in 1983, thought this major work provided a solid paradigm for psychological research, as he viewed psychology as a pre-paradigmatic scientific discipline (T.S. Kuhn had used the term "pre-paradigmatic" for social science).