What the Mind Does: Internalization and Externalization
The concepts of internalization and externalization are found with some frequency in psychology. It is said that the child internalizes the surrounding culture, including moral and social norms (the same can be said of an adult transplanted into a hitherto alien culture). It is also said that the child internalizes the rules of grammar of the language he or she grows up to speak. In clinical psychology we hear that a person has internalized family conflicts or role models or patterns of response. On the other hand, psychoanalysts have suggested that people externalize their own psychological traits, as with classic projection; and it can be added that animism and anthropomorphism are instances of the same phenomenon. Linguists speak of public communicative language as the externalization of an (innate) internal language system.  Art and technology are sometimes held to be externalizations of the mind. These are certainly suggestive ways of talking, but what do they really mean? How literally should they be taken? Can they be elevated into a general theory of the mind?
As always the dictionary provides a useful starting point. The OED gives us this for “internalize”: “make (attitudes or behavior) part of one’s nature by learning or unconscious assimilation; acquire knowledge of (the rules of language)”. For “externalize” we have: “give external existence or form to; project (a mental image or process) onto a figure outside oneself”. Merriam-Webster has: “to make external or externally manifest”. The idea of internalization is that something originally external to the mind is rendered part of the nature of what is internal to the mind: it was “out there” and now it is “in here”, shaping it, forming its inner architecture. Thus what was once merely the code of one’s society becomes one’s own inner code—a sort of cultural invasion has occurred (often unconsciously). The language spoken by others is incorporated into one’s own symbolic faculty, made internal. The mind is evidently capable of this internalization operation, converting what is perceived externally into a feature of one’s interior landscape. In the case of externalization the governing idea is that what is originally inner is expressed outwardly: the external comes to have the form of the internal—it is the internal made manifest. The nature of the external thus reflects the nature of the internal. Evidently the mind is capable of this feat of externalization, converting mind into world, inner into outer. The internal is made external, as the external can be made internal. The mind pushes outward as it also pulls inward. Notice that this is not the same concept as the concept of causation: it is not merely that the external can cause the internal and the internal can cause the external; rather, the external can determine the nature of the internal and the internal can leave its imprint on the external. We might say that there is in both cases an “internal relation” between the inner and the outer: each side reflects the other. A sort of isomorphism obtains, as well as a kind of dependence. This is a strong relation, in which the mind is said to literally internalize what lies outside its boundaries, as well as literally to externalize what is within it. Clearly an exceptionally tight and intimate relationship is envisaged, a kind of overlapping of internal and external.
It might be wondered whether others parts of nature can be said to internalize and externalize. The idea is not preposterous; indeed, the term “internalize” is used in biology to describe the cell pulling external material across its membrane and into its interior (“endocytosis”). The justification for using this term is obviously that a boundary exists across which certain items flow, so changing the nature of the entity they flow into. There is the internal landscape of the cell and there is its external environment, and the latter can penetrate the former, molding it in the process. I suppose we could extend the use of the term to feeding: the external in the form of food is taken in and converted into tissues of the body, shaping them—though here the idea of changing the nature of the body seems strained. But the abstract notion of an entity with boundaries being shaped by and shaping its environment, by dint of a transfer of elements, seems generally applicable. Even in physics and chemistry we could justify talking this way, though to my knowledge physicists and chemists don’t: the atom absorbs and expels energy, internalizing and externalizing a transferable ingredient; heat is absorbed into a system altering its behavior, also seeping out to alter the surrounding world; osmosis is the passage of molecules from inside a physical system to outside it (and vice versa). There is what is internal to an entity and what is external to it; and the activity of the entity, in conjunction with its environment, involves various kinds of internalization and externalization. We might, indeed, contrive a metaphysical system from this basic structure: The world is the totality of internalizations and externalizations. There are these entities called “monads” and they operate by internalizing what is around them and externalizing what is within them. They are not isolated atoms but essentially interacting entities whose nature is fixed by acts of internalization and externalization. There is a dialectic between internalization and externalization that defines the ontological structure of reality. We might even conjoin this metaphysical picture with panpsychism: the psychic units that constitute so-called physical reality internalize what exists around them and they also externalize themselves in the form of perceptible matter. The metaphysical possibilities are endless: history is the externalization of the internalized; God internalizes everything and his creation is the externalization of his nature; mind is matter internalized and matter is mind externalized…
Putting that metaphysical flurry aside, we can focus on the mind and its capacity to engage in both sorts of operation. Clearly we must assume some sort of boundary in order to make sense of the concepts of internalization and externalization. The external must lie on the further side of this boundary in order to be capable of crossing it from elsewhere. The mind must be bounded by something or else there would be nothing external to it. This boundary could be conceived in many ways, depending on further ontological commitments: it might just be the boundary of the brain or the body, or it might be conceived immaterially. A dualist would suppose that the immaterial substance could internalize extended substance by that substance crossing an immaterial boundary—not presumably by actual spatial transfer but by some sort of extraction of form. Similarly, the mental substance could externalize itself by imparting its form to material substance: but not by itself becoming material. In the case of language the talk of externalization is motivated by the idea that both the internal language and its outer expression share their grammatical structure (and maybe lexicon), so that we can say that the internal structure is manifested in the structure of outer speech. Thus outer public language has a derivative structure determined by copying the original structure of the internal language (perhaps supplemented by other sources of structure). Likewise, if we thought that inner speech were the internalization of outer speech, we would suppose that the derivation goes the other way. On either view we have an isomorphism of grammatical structure. We don’t tend to think this way about knowledge in general, as if all knowledge is a kind of internalization of the outer—as if knowledge of the universe were the internalization of the universe. We internalize the rules of English, say, but we don’t internalize the planets (those rocky entities). But that may be a reflection of a misguided internalist view of the mind: wouldn’t an externalist say that (some) mental states are (partly) individuated by objects in the environment, so that it is acceptable to say that the mind internalizes the external world? On earth we internalize water (H2O) in our thoughts and meanings, while on twin earth we internalize retaw (XYZ). So we could in principle extend the idea of knowledge-as-internalization to the full range of knowledge, not just knowledge of language or cultural norms. We internalize objects and facts as well as rules and attitudes. 
Similarly for externalization: why should we not extend the idea of externalization further than is customary? Why not view all behavior as the externalization of the mind? An action is an intention externalized. Art and artifacts count as externalizations, so what about the actions that lead to them? The notion of expressionencourages this thought: we express our emotions in our body, and this expression is surely a form of externalization (consider facial expressions). There is a natural “fit” between inner feeling and its bodily expression, not merely a causal connection. Just as perception can be viewed as the internalization of objects, so action can be viewed as the externalization of desires (etc.). The mind takes in and it also gives out. It isn’t just that reasons cause actions; actions externalize reasons—embody them, lend them material form (consider a chess move). And it isn’t just that objects cause perceptions; perceptions internalize objects. The relation is a lot more intimate and internal than we have tended to suppose—not logical perhaps, but certainly structural.  Maybe it is a general property of mind to be an internalizing externalizer. The mind absorbs things across its boundary and it extrudes things in the opposite direction. For instance, the mind internalizes the rules of grammar to achieve mastery of a particular language, but this mastery is externalized in actual speech. In the case of innate knowledge of the universal rules of grammar, there is no such internalization, since the knowledge is present ab initio; but there is still externalization as the internal language faculty hooks up with sensorimotor systems. It is natural to suppose that there exists an innate internal mental apparatus prior to any internalization of the environment, and that this apparatus interacts with the environment to lead to internalized knowledge; this composite system then interlocks with sensorimotor systems to make externalization possible—spoken language and maybe action in general (as well as art and technology). We have a transition from the internal to the internalized to the externalized. There are internal and external domains and there are operations that cross these boundaries, thus producing mixed domains of the internalized and the externalized. Not everything in the internal domain is internalized (we are not empiricists) and not everything in the external domain is externalized (we are not idealists): but there is a good deal of internalized and externalized stuff in the world—intersections of the internal and the external mediated by the operations of internalization and externalization.
People speak of stimulus-response psychology and of computational psychology to characterize a general conception of how the mind works and what it does; we can likewise speak of internalization-externalization psychology (or I-E psychology) to capture the general conception of how the mind works we are exploring. This is quite a specific conception, incorporating as it does the idea that the mind is a device for internalization and externalization—not for mediating stimulus and response or for performing computations (though these ideas need not be regarded as simply false, just incomplete). It depicts the mind in a particular way—not just as an input-output device, but as something that performs a characteristic kind of operation of conversion. Not that the conversion operations in question are well understood or free of mystery; indeed, they are quite puzzling. For how is it possible to internalize the external or to externalize the internal? Isn’t this contradictory? It seems like a peculiar form of mental alchemy—yet it is evidently what happens. Nor can we draw comfort from those non-psychological analogues I mentioned earlier, because abstract similarity is not identity of mechanism: how the mind contrives to internalize and externalize is left unexplained. It is not just a matter of absorbing molecules across a membrane or emitting energy from an atom; these are specifically psychological processes—a sort of mimicry, perhaps, whereby the outer is converted to the inner and the inner is converted to the outer. It is not like inserting a marble into a box or ejecting a marble from a box, in which the ideas of internalization and externalization have literal spatial meaning; but it is not entirely unlike that either. It is as if the external world is inserted into the mind, or extruded from it. People speak of the extended mind; well, this is the externalized mind. Similarly, we can speak of the “internalized world” to capture the power of the mind to bring the world into its domain (the world extends beyond its own natural boundaries to take up residence in minds). The mind protrudes into the world in acts of externalization, but the world also protrudes into the mind in acts of internalization. Each visits the other’s territory, leaving its distinctive mark. That is the essence of the mental—internalizing and externalizing, crossing an interface.
Let me illustrate how this conceptual framework applies by considering two unrelated topics: music and sense. Music is closely associated with the emotions, and this gives it a unique place in the operations of internalization and externalization. On the one hand, music is easily internalized, as if it is designed to be: we hear a tune with our outer ear and instantly it enters our inner musical landscape. Its emotional resonance plays a role in this ready internalization. On the other hand, audible music expresses inner feeling perfectly, so lending itself to the externalization of emotion—in the form of dancing, singing, playing an instrument, humming, etc. We internalize tunes and we externalize what is thus internalized. And there is an especially intimate connection between the internal and the external in the case of music: we repeat the external tune silently in our head, and we externalize our inner experience in forms that precisely mirror what is going on internally (notably when singing). The psychology of music is thus steeped in internalization and externalization, and would hardly be conceivable without them. In the case of sense, Frege supposed that senses are objective external entities, but he also supposed that they shape the very nature of thoughts: we internalize senses in such a way that they enter our cognitive landscape (the same could not be said of references as Frege conceived them). At the same time grasp of sense is externalized in public symbolic systems—these systems make sense manifest. So senses are both internalized and externalized—and this is essential to their identity, what they are. If we think of senses as existing in our environment (in some extended sense), then the task of thought is to internalize them; but once internalized they are available for externalization in language, so that language becomes sense externalized. That is the psychology of sense: first internalization, then externalization—internal absorption and external expression. And that is psychology generally: internalizing the world and then externalizing what has been made internal. It isn’t just that an outer stimulus elicits an overt response via an internal process, or that information flows from the environment into the brain and then out to behavior. Rather, the mind possesses a specific set of capacities that we call “internalization” and “externalization”; and these capacities deserve the names they have been given. If we want a single word to express this general conception, analogous to “behaviorism” and “computationalism”, we might choose “conversionism” for want of anything better. 
 This is the position taken by Chomsky: the primary instantiation of language is as an internal system connected initially to thought; only later is language hooked up to sensorimotor systems that produce a spoken communicative language. When that happens the internal system is externalized, i.e. its properties are transferred to an outer capacity. Outer language has structure because inner language has structure—the former is derivative from the latter. See Why Only Us, Robert C. Berwick and Noam Chomsky (MIT Press, 2016).
 I don’t say we should endorse this proposal, only that it is worth considering. An alternative would be to divide all knowledge into two classes: internalized knowledge and non-internalized knowledge. Then we could have debates about where given types of knowledge fall—what about knowledge of mathematics or ethics? Similarly, there will be a division between externalized facts about the world and non-externalized facts (which are presumably much more numerous, if we keep God out of it).
 One approach would be that perception involves building a mental model of the perceived object, thus replicating its structure internally. Or we might go full externalist and insert the object itself into perceptual content, expanding the mind into physical space. And there are other approaches too that could be used to justify the word “internalize”.