•Around the same time as Pavlov worked in Russia,E.L.Thorndike in the US was working on cats, dogs and chicks to understand the learning process.•He differed in one major way from Pavlov.•Thorndike placed his animals in a "puzzle box" from which they were required to escape, by manipulating the correct lever or pulley, in order to obtain food reward.•Through the process of trial and error, Thorndike's animals eventually "learned".•Since the ANIMAL'S BEHAVIOR WERE INSTRUMENTAL in escaping from the box and in obtaining food, the paradigm was labeled "Instrumental Conditioning".•For Thorndike, response was more important for learning than the stimulus.•His theory can , therefore, be called RESPONSE GENERALIZATION THEORY.