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.
Philosophy of Søren Kierkegaard
The philosophy of Søren Kierkegaard has been a major influence in the development of 20th-century philosophy, especially existentialism and postmodernism. Kierkegaard was a 19th-century Danish philosopher who has been labeled by many as the "Father of Existentialism", although there are some in the field who express doubt in labeling him an existentialist to begin with. His philosophy also influenced the development of existential psychology.