According to mentalist theories, acquisition is seen as a product of the complex interaction of the linguistic environment and the learner's internal mechanisms, that is, input does have a determining function in language acquisition, but only within the constraints imposed by the leaner's internal mechanisms.

The mentalist view emphasizes the possibility of shaping L2 acquisition by manipulating the input to provide appropriate stimuli by ensuring that adequate feedback is always available. Acquisition is thus controlled by external factors, and the learner is viewed as a passive medium.

In cognitive interactionist models, feedback takes the form of positive reinforcement or correction, depending on whether the learner's output is perceived to be target-like.

Behaviourist accounts of L2 acquisition propose a direct relationship between input and output. Because they reject an idea of "mind" as an object for inquiry, they ignore the internal processing that takes place inside the learner.