Complex sentence: Concurrently thriving alongside mentalism since the inception of psychology was the idea of behaviorism, but it was not until 1913, when psychologist John B. Watson published his article "Psychology as the Behaviorist Views It" that it began to steal the spotlight from classical mentalism.
Split into simpler sentences: Concurrently thriving alongside mentalism since the inception of psychology was the perspective of behaviorism. However, it was not until 1913, when psychologist John B. Watson published his article "Psychology as the Behaviorist Views It" that behaviorism began to steal the spotlight from classical mentalism.