Internalism and externalism are now part of the standard jargon of philosophical discourse, and are central to important debates. The distinction crops up in many areas with similar but distinct meanings.
In contemporary moral philosophy, motivational internalism is the view that moral beliefs (or judgments) are intrinsically motivating. That is, the motivational internalist believes that there is an internal, necessary connection between one's belief that X ought to be done and one's motivation to do X. Conversely, the motivational externalist claims that there is no necessary, internal connection between moral beliefs (or judgments) and moral motives. That is, there is no necessary connection between the belief that X is wrong and the desire not to do X. (The use of these terms has roots in W.D. Falk's paper "'Ought' and Motivation" (1947).)
These views in moral psychology have various implications. In particular, if motivational internalism is true, then an amoralist 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 the right thing to 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 (Rosati, 2006).
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 (Finlay & Schroeder, 2008, §1.1). 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 to not 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 (1981), is that reasons for action must be able to explain one's action; and only internal reasons can do this.
In contemporary epistemology internalism about justification is the idea that everything necessary to provide justification for a belief is immediately available in consciousness. Externalism in this context is the view that there are factors other than those which are internal to the believer which can affect the justificatory status of a belief. One strand of externalism is loosely called the causal theory of knowledge, and reliabilism is sometimes considered to be another strand. It is important to distinguish internalism about justification from internalism about knowledge. An internalist about knowledge will likely hold that the conditions that distinguish mere true belief from knowledge are similarly internal to the individual's perspective or grounded in the subject's mental states. Whereas externalism about justification is a widely endorsed view, there are few defenders of externalism about knowledge thanks in no small part to Edmund Gettier and Gettier-examples that suggest that there is more to knowledge than just justified true belief. In a short but widely discussed paper published in 1963, Gettier produced examples that seemed to show that owing to an accidental connection between an individual's evidence or reasons and the truth of their belief, someone could be justified in believing something true but nevertheless be ignorant.
One line of argument in favor of externalism begins with the observation that if what justified our beliefs failed to significantly eliminate the risk of error, then it does not seem that knowledge would be attainable as it would appear that when our beliefs did happen to be correct, this would really be a matter of good fortune. While many will agree with this last claim, the argument seems inconclusive. Setting aside sceptical concerns about the possession of knowledge, Gettier cases have suggested the need to distinguish justification from warrant where warrant is understood as that which distinguishes justified true belief from knowledge by eliminating the kind of accidentality often present in Gettier-type cases. Even if something must significantly reduce the risk of error, it is not clear why justification is what must fill the bill.
One of the more popular arguments for internalism begins with the observation, perhaps first due to Stewart Cohen, that when we imagine subjects completely cut off from their surroundings (thanks to a malicious Cartesian demon, perhaps) we do not think that in cutting these individuals off from their surroundings, these subjects cease to be rational in taking things to be as they appear. The 'new evil demon' argument for internalism (and against externalism) begins with the observation that individuals like us on the inside will be as justified as we are in believing what we believe. As it is part of the story that these individuals' beliefs are not produced by reliable mechanisms or backed by veridical perceptual experiences, the claim that the justification of our beliefs depends upon such things appears to be seriously challenged. Externalists have offered a variety of responses but there is no consensus among epistemologists as to whether these replies are successful (Cohen, 1984; Sosa, 1991).
To clarify how this argument is supposed to work: Imagine that there is brain in a vat, and a whole world is being simulated for it. Call the individual who is being deceived "Steve." When Steve is given an experience of walking through a park, semantic externalism allows for his thought, "I am walking through a park" to be true so long as the simulated reality is one in which he is walking through a park. Similarly, what it takes for his thought "I am a brain in a vat" to be true is for the simulated reality to be one in which he is a brain in a vat. But in the simulated reality, he is not a brain in a vat.
Apart from disputes over the success of the argument or the plausibility of the specific type of semantic externalism required for it to work, there is question as to what is gained by defeating the skeptical worry with this strategy. Skeptics can give new skeptical cases that wouldn't be subject to the same response (e.g. one in which the person was very recently turned into a brain in a vat, so that their words "brain" and "vat" still pick out real brains and vats, rather than simulated ones). Further, if even brains in vats can correctly believe "I am not a brain in a vat," then the skeptic can still press us on how we know we are not in that situation (though the externalist will point out that it may be difficult for the skeptic to describe that situation).
Another attempt to use externalism to refute skepticism is done by Brueckner and Warfield. It involves the claim that our thoughts are about things, unlike a BIV's thoughts which cannot be about things (DeRose, 1999).
Semantic externalism comes in two varieties, depending on whether meaning is construed cognitively or linguistically. On a cognitive construal, externalism is the thesis that what concepts (or contents) are available to a thinker is determined by their environment, or their relation to their environment. On a linguistic construal, externalism is the thesis that the meaning of a word is environmentally determined. Likewise, one can construe semantic internalism in two ways, as a denial of either of these two theses.
Externalism and internalism in semantics is closely tied to the distinction in philosophy of mind concerning mental content, since the contents of one's thoughts (specifically, intentional mental states) are usually taken to be semantic objects that are truth-evaluable.
Within the context of the philosophy of mind, externalism is the theory that the contents of at least some of one's mental states are dependent in part on their relationship to the external world or one's environment. Some mental states, such as believing that water is wet, and fearing that the Queen has been insulted, have contents which we can capture using 'that' clauses. The content externalist often appeals to observations which are found as early as Hilary Putnam's seminal essay "The Meaning of 'Meaning'" (1975) that we can easily imagine pairs of individuals that are microphysical duplicates embedded in different surroundings who use the same words but mean different things when using them. For example, suppose that Ike and Tina's mothers are identical twins and that Ike and Tina are raised in isolation from one another in indistinguishable environments. When Ike says 'I want my mommy', he expresses a want that is satisfied only if he is brought to his mommy. If we brought Tina's mommy, Ike might not notice the difference, but he doesn't get what he wants. It seems that what he wants and what he says when he says 'I want my mommy' will be different from what Tina wants and what she says she wants when she says 'I want my mommy'. Externalists say that if we assume that competent speakers know what they think and say what they think, the difference in what these two speakers mean will correspond to a difference in the minds of the two speakers that is not (necessarily) reflected by an internal difference in the internal make up of the speakers or thinkers. They urge us to move from externalism about meaning of the sort Putnam defended to externalism about contentful states of mind. The example offered pertains to singular terms but it has been extended to cover kind terms as well such as natural kinds (e.g., 'water') and for kinds of artifacts (e.g., 'espresso maker'). There is no general agreement amongst content externalists as to the scope of the thesis.
Philosophers now tend to distinguish between wide content (externalist mental content) and narrow content (anti-externalist mental content). Some, then, align themselves as endorsing one view of content exclusively, or both. For example, Jerry Fodor (1980) argues for narrow content (although he comes to reject that view in his 1995), while David Chalmers (2002) argues for a two dimensional semantics according to which the contents of mental states can have both wide and narrow content.
Critics of the view have questioned the original thought experiments saying that the lessons that Putnam and later writers such as Tyler Burge (1979, 1982) have urged us to draw can be resisted. Frank Jackson and John Searle, for example, have defended internalist accounts of thought content according to which the contents of our thoughts are fixed by descriptions that pick out the individuals and kinds that our thoughts intuitively pertain to the sorts of things that we take them to. In the Ike/Tina example, one might agree that Ike's thoughts pertain to Ike's mother and that Tina's thoughts pertain to Tina's but insist that this is because Ike thinks of that woman as his mother and we can capture this by saying that he thinks of her as 'the mother of the speaker'. This descriptive phrase will pick out one unique woman. Externalists have claimed that this is implausible as we would have to ascribe Ike knowledge he wouldn't seem to need in order to successfully refer to his mother or think about his mother.
Critics have also claimed that content externalists are committed to epistemological absurdities. Suppose that a speaker can have the concept of water we do only if the speaker lives in a world that contains H2O. It seems this speaker could know a priori that she thinks that water is wet. This is the thesis of privileged access. It also seems that she could know on the basis of simple thought experiments that she can only think that water is wet if she lives in a world that contains water. What would prevent her from putting these together and coming to know a priori that the world contains water? If we should say that no one could possibly know whether water exists a priori, it seems either we cannot know content externalism to be true on the basis of thought experiments or we cannot know what we are thinking without first looking into the world to see what it is like.
Externalism in the historiography of science is the view that the history of science is due to its social context - the socio-political climate and the surrounding economy determines scientific progress.
Internalism in the historiography of science claims that science is completely distinct from social influences and pure natural science can exist in any society and at and time given the intellectual capacity.