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 certain general principles, which have been developed in general theories of process learning. These include operant conditioning and classical conditioning which were put forward by eminent psychologists like Pavlov, BFSkinner and Thorndike. However, in learning, operant and classical conditioning are opposed by biological constraints which stipulate that there are limits to the theories. Some of these biological constraints on learning will be discussed below. Operant conditioning learning allows conditioned behavior to increase or decrease in the presence of reinforcement or punishment. However, this process can be affected by instinctive behaviors that disrupt conditioned behavior. According to a study 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 some time rubbing the coins together and rubbing the coin inside the container before finally dropping it and receiving its food reinforcement. Even after conditioning, the raccoon's need to rub the pieces together became worse as it spent more and more time simply rubbing the pieces. This is called instinctive drift, where the raccoon's instinctive behaviors limited its ability to execute the conditioned response. Therefore, the raccoon failed to learn due to its innate tendencies which acted 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 dogs. ... middle of article ......show that there are limits to learning theories. But Domjan and Galef's (1983) studies of constraints on learning suggest that there is no basis per se on which general theories of process learning would not be viable. They believe this is because the study of operant and classical conditioning now includes many phenomena that were not initially incorporated into the theories. Domjan and Galef (1983) argue that although biological constraints have not prevailed as such, much research on the theory of constraints has derived key empirical results for understanding general theories of process learning. Therefore, according to the above, a general theory of process learning is sustainable even in the presence of biological constraints, because behavior can be reinforced and manipulated in most cases to acquire the desired behavior.