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.
History of psychology
The Danish philosopher Søren Kierkegaard also influenced the humanistic, existential, and modern psychological schools with his works "The Concept of Anxiety" (1844) and "The Sickness Unto Death" (1849).