Avram Noam Chomsky (1928-present) is known as the father of modern linguistics, and is one of the most staunch defenders of cognitive theory as it applies to second language acquisition and overall learning.
A life-long follower of the cognitive school of thought, Chomsky supports the claim that neural processes taking place in specific parts of the brain are responsible for the development of skills. These acquired skills can range from Math, to Writing, to Second language learning. In theory, learning is acquired once patterns are perceived and information is processed.
There is a lot to consider when proposing a cognitive approach to teaching and learning. Mainly, that the bio-psychological, developmental, and neural processes of the learner are in optimal condition to undergo pattern recognition and information processing. When these conditions are not met, there are still other processes that take place in the brain that can be molded through neuroplasticity so that less-developed students can still acquire language. In all, cognitive theory entails that all humans have an inherent ability to process information that works naturally when the right conditions are met.
In a direct contrast with cognitive theory, Behaviorism is a learning theory that supports the claim that all learning is a combination of stimulus and response; that teaching and learning follow an input-output model from which the student acquires information through habitual practices and repetition. B.F. Skinner is to behaviorism what Chomsky is to cognitive theory. In 1957, Skinner wrote a treaty titled Verbal Behavior. The article was the conduit that concreted his philosophical views; that every mental process is a result of a habitually-learned behavior. Skinner gives little value to inherent cognitive activities as the main agents of learning. It is not that Skinner does not believe that mental processes lead to learning. What he means is that these mental processes occur, or are "activated", as a result of specific patterns of activity during learning.
For example, a behaviorist would agree in that a student cannot fully learn unless he repeats the information over and over and then regurgitates it right back. To a cognitive theorist, a student can learn casually and without specific habits via the natural processes that take place in our brains.
In his article "A Review of B. F. Skinner's Verbal Behavior", Chomsky sets the record straight: he is highly opposed to behaviorist theory for many reasons.
The first of Chomsky's arguments is that behavioral testing, such as it is shown in the studies of Skinner, has been mainly used to test animals. Moreover, behavioral theory does not consider the variables that exist in the behavior of humans, who are much more complex creatures. As a result, Chomsky argues that behaviorism attempts to study activity
involving higher mental faculties within a strict behaviorist schema
This behavioral schema is very stiff: it is based on stimulus, response, and reinforcement. It is sterile in the manner in which it treats the contrivances of everyday dynamics, and it fails to recognize much needed variables in development, intellectual adeptness, motivation, and skill application. The argument is based on the impossibility to award human behavior a predictable rule of behavior due to the complexity of base-by-base adaptability. These are Chomsky's main complaints.