Dr. Aaron T. Beck, whose pragmatic, thought-monitoring psychotherapy became the centerpiece of a scientific transformation in the treatment of depression, anxiety and other related mental disorders, died Dec. 1 at his home in Philadelphia. He was 100.

Dr. Beck was a young psychiatrist trained in Freudian analysis when, in the late 1950s, he began prompting patients to focus on distortions in their day-to-day thinking, rather than on conflicts buried in childhood, as therapists typically did. He discovered that many people generated what he called “automatic thoughts,” unexamined assumptions like “I’m just unlucky in love” or “I’ve always been socially inept,” which can give rise to self-criticism and despair.

Dr. Beck found that he could undermine those assumptions, gradually improving people’s mood. His work provided the architecture for what is known as cognitive behavior therapy, which has become the world’s most extensively studied form of psychotherapy.