Internalism and externalism are two opposite ways of integrating and explaining various subjects in several areas of philosophy. These include human motivation, knowledge, justification, meaning, and truth. The distinction arises in many areas of debate with similar but distinct meanings. Internal–external distinction is a distinction used in philosophy to divide an ontology into two parts: an internal part concerning observation related to philosophy, and an external part concerning question related to philosophy.

Internalism is the thesis that no fact about the world can provide reasons for action independently of desires and beliefs. Externalism is the thesis that reasons are to be identified with objective features of the world.

In contemporary moral philosophy, motivational internalism (or moral internalism) is the view that moral convictions (which are not necessarily beliefs, e.g. feelings of moral approval or disapproval) are intrinsically motivating. That is, the motivational internalist believes that there is an internal, necessary connection between one's conviction that X ought to be done and one's motivation to do X. Conversely, the motivational externalist (or moral externalist) claims that there is no necessary internal connection between moral convictions and moral motives. That is, there is no necessary connection between the conviction that X is wrong and the motivational drive not to do X. (The use of these terms has roots in W.D. Falk's (1947) paper "'Ought' and Motivation".)

These views in moral psychology have various implications. In particular, if motivational internalism is true, then amorality is unintelligible (and metaphysically impossible). An amoralist is not simply someone who is immoral, rather it is someone who knows what the moral things to do are, yet is not motivated to do them. Such an agent is unintelligible to the motivational internalist, because moral judgments about the right thing to do have built into them corresponding motivations to do those things that are judged by the agent to be the moral things to do. On the other hand, an amoralist is entirely intelligible to the motivational externalist, because the motivational externalist thinks that moral judgments about what is right do not necessitate some motivation to do those things that are judged to be the right thing to do; rather, an independent desire—such as the desire to do the right thing—is required (Brink, 2003), (Rosati, 2006).

There is also a distinction in ethics and action theory, largely made popular by Bernard Williams (1979, reprinted in 1981), concerning internal and external reasons for an action. An internal reason is, roughly, something that one has in light of one's own "subjective motivational set"—one's own commitments, desires (or wants), goals, etc. On the other hand, an external reason is something that one has independent of one's subjective motivational set. For example, suppose that Sally is going to drink a glass of poison, because she wants to commit suicide and believes that she can do so by drinking the poison. Sally has an internal reason to drink the poison, because she wants to commit suicide. However, one might say that she has an external reason not to drink the poison because, even though she wants to die, one ought not to kill oneself no matter what—regardless of whether one wants to die.

Some philosophers embrace the existence of both kinds of reason, while others deny the existence of one or the other. For example, Bernard Williams (1981) argues that there are really only internal reasons for action. Such a view is called internalism about reasons (or reasons internalism). Externalism about reasons (or reasons externalism) is the denial of reasons internalism. It is the view that there are external reasons for action; that is, there are reasons for action that one can have even if the action is not part of one's subjective motivational set.

Consider the following situation. Suppose that it's against the moral law to steal from the poor, and Sasha knows this. However, Sasha doesn't desire to follow the moral law, and there is currently a poor person next to him. Is it intelligible to say that Sasha has a reason to follow the moral law right now (to not steal from the poor person next to him), even though he doesn't care to do so? The reasons externalist answers in the affirmative ("Yes, Sasha has a reason not to steal from that poor person."), since he believes that one can have reasons for action even if one does not have the relevant desire. Conversely, the reasons internalist answers the question in the negative ("No, Sasha does not have a reason not to steal from that poor person, though others might."). The reasons internalist claims that external reasons are unintelligible; one has a reason for action only if one has the relevant desire (that is, only internal reasons can be reasons for action). The reasons internalist claims the following: the moral facts are a reason for Sasha's action not to steal from the poor person next to him only if he currently wants to follow the moral law (or if not stealing from the poor person is a way to satisfy his other current goals—that is, part of what Williams calls his "subjective motivational set"). In short, the reasoning behind reasons internalism, according to Williams, is that reasons for action must be able to explain one's action; and only internal reasons can do this.

Two main varieties of epistemic internalism about justification are access internalism and ontological internalism. Access internalists require that a believer must have internal access to the justifier(s) of their belief p in order to be justified in believing p. For the access internalist, justification amounts to something like the believer being aware (or capable of being aware) of certain facts that make her belief in p rational, or them being able to give reasons for her belief in p. At minimum, access internalism requires that the believer have some kind of reflective access or awareness to whatever justifies her belief. Ontological internalism is the view that justification for a belief is established by one's mental states. Ontological internalism can be distinct from access internalism, but the two are often thought to go together since we are generally considered to be capable of having reflective access to mental states.