Learning is defined as a “process of change that occurs as a result of an individual's experience” (Mazure, 2006). Researchers assume that the learning process follows some general principles, which have been developed into general theories of the learning process. These include operant conditioning and classical conditioning which was proposed by leading psychologists such as Pavlov, BFSkinner and Thorndike. However, in learning, operant and classical conditioning oppose biological constraints that state that there are limitations to theories. Some of these biological constraints on learning will be discussed below. Learning through operant conditioning allows a conditioned behavior to increase or decrease in the presence of reinforcement or punishment. However, this process can be influenced by instinctive behaviors that interrupt conditioned behavior. According to a study conducted by Breland and Breland (1961) they tried to condition a raccoon to pick up coins and drop them into a container. The raccoon, however, spent time rubbing the coins together and rubbing the coin inside the container before dropping it and receiving his food reinforcer. Even after conditioning, the raccoon's need to rub the coins worsened as he spent more and more time simply rubbing the coins. This is known as instinctive drift where the raccoon's instinctive behaviors limit its ability to perform the conditioned response. Therefore the raccoon failed to learn due to its innate tendencies acting as a biological constraint and operant conditioning failed to teach the raccoon through reinforcement. In the same study by Breland and Breland (1961) a pig was conditioned to pick up... ... half of the paper ...... shows that there are limitations to learning theories. But Domjan and Galef's (1983) studies of learning constraints suggest that there is no basis as such on which general theories of the learning process are untenable. According to them this is because the study of both operant and classical conditioning now includes many of the phenomena that were not initially incorporated into the theories. Domjan and Galef (1983) state that, although biological constraints have not prevailed as such, many of the investigations into the theory of constraints have yielded key empirical findings for understanding general theories of process learning. Therefore, according to the above, a general theory of process learning is tenable even in the presence of biological constraints since behavior can be reinforced and manipulated in most cases to acquire the desired behavior.