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.
Modern Stoicism
Stoic philosophy was the original philosophical inspiration for modern cognitive psychotherapy, particularly as mediated by Dr Albert Ellis' Rational-Emotive Behaviour Therapy (REBT), the major precursor of CBT. The original cognitive therapy treatment manual for depression by Aaron T. Beck et al. states, "The philosophical origins of cognitive therapy can be traced back to the Stoic philosophers". A well-known quotation from "The Handbook" of Epictetus was taught to most clients during the initial session of traditional REBT by Ellis and his followers: "It's not the events that upset us, but our judgments about the events." This subsequently became a common element in the "socialization" phase of many other approaches to CBT. The question of Stoicism's influence on modern psychotherapy, particularly REBT and CBT, was described in detail in "The Philosophy of Cognitive-Behavioural Therapy" (2010) by Donald Robertson. Moreover, several early 20th century psychotherapists were influenced by stoicism, most notably the "rational persuasion" school founded by the Swiss neurologist and psychotherapist Paul DuBois, who drew heavily on Stoicism in his clinical work and encouraged his clients to study passages from Seneca as homework assignments.