Ivan Pavlov, 2. B.F. Skinner, and 4. John Watson all agreed that psychology should be solely concerned with objective, observable behavior. Pavlov was a Russian physiologist who is known for his work on classical conditioning, in which he demonstrated that animals can learn to associate certain stimuli with specific responses. Skinner was an American psychologist who developed the theory of operant conditioning, which states that behavior is determined by its consequences. Watson was an American psychologist who is known for his work in the field of behaviorism, which emphasizes the study of observable behavior rather than mental processes