Internalism is the view in Epistemology that everything necessary to provide justification for a belief is immediately available in a person's consciousness without having to resort to external factors, or at least that these things are cognitively accessible to a person. Therefore, all knowledge-yielding conditions are within the psychological states of those who gain knowledge. It is a type of Psychologism (any position in which psychology plays a central role in grounding or explaining some other, non-psychological type of fact or law).

The alternative view is known as Externalism (the view that there are factors, other than those which are internal to the believer, which can affect the justificatory status of a belief).

Internalism argues that if we were to imagine subjects completely cut off from their surroundings, these subjects do not then necessarily cease to be rational in taking things to be as they appear. Thus, a pair of psychological twins (two agents with identical mental states) will be equally justified in all of their beliefs, and indeed they will have all the same beliefs.

Opponents of the doctrine have argued (along lines similar to the famous Gettier Cases of Edmund Gettier, combined with a version of Descartes' evil demon argument) that if one hypothetical psychological twin obtains his belief in something (say, that a clock is showing 12 o'clock midnight) from looking at a good working clock, and the other twin has exactly the same belief but that belief is brought about purely by an evil demon, then the justification of the belief has been influenced by something external to the mental states of the twins.

• Semantic Internalism is the thesis that the concepts available to individuals (or, in a linguistic construal, the meanings of words) are not determined by the environment of those individuals or their relation to the external world.
• Motivational Internalism is the view in Ethics and moral psychology that moral beliefs or judgments are intrinsically motivating, and that there is an internal, necessary connection between a person's belief that something ought to be done and their motivation to do it. Thus, amorality (the lack of moral sensibility, not caring about right and wrong) is unintelligible and even impossible to a motivational internalist.
• Historiographical Internalism in Philosophy of Science claims that science is completely distinct from social influences and pure natural science can exist in any society and at any time, given the intellectual capacity.