A key paper by Tolman, Ritchie, and Kalish in 1946 demonstrated that rats learned the layout of a maze, which they explored freely without reinforcement. After some trials, a food item was placed to a certain point of the maze, and the rats learned to navigate to that point very quickly. However, Hull and his followers were able to produce alternative explanations of Tolman's findings, and the debate between S-S and S-R learning theories became increasingly complicated. Skinner's iconoclastic paper of 1950, entitled ""Are theories of learning necessary?"", persuaded many psychologists interested in animal learning that it was more productive to focus on the behavior itself rather than using it to make hypotheses about mental states. The influence of Tolman's ideas faded temporarily in the later 1950s and 1960s. However, his achievements had been considerable. His 1938 and 1955 papers, produced to answer Hull's charge that he left the rat "buried in thought" in the maze, unable to respond, anticipated and prepared the ground for much later work in cognitive psychology, as psychologists began to discover and apply decision theory – a stream of work that was recognized by the award of a Nobel prize to Daniel Kahneman in 2002. In his 1948 paper ""Cognitive Maps in Rats and Men"", Tolman introduced the concept of a cognitive map, which has found extensive application in almost every field of psychology, frequently among scientists who are unaware that they are using the early ideas that were formulated to explain the behavior of rats in mazes. Tolman assessed both response learning and place learning. Response learning is when the rat knows that the response of going a certain way in the maze will always lead to food; place learning is when the rats learn to associate the food in a specific spot each time. In his trials he observed that all of the rats in the place-learning maze learned to run the correct path within eight trials and that none of the response-learning rats learned that quickly, and some did not even learn it at all after seventy-two trials.