Externalism and the Cogito
The basic meaning of the Cogito is that the existence of a conscious state implies the existence of a subject of that state. Thus thinking implies a thing that thinks, where that thing is not identical to the thought itself. We might say that something external to the thought is implied by the thought—a distinct existence. If selves were material objects, this would give us the result that conscious states imply the existence of material objects—though that is not the only conception of the self that might be invoked. Still, the idea is that something of another nature is implied by consciousness: it isn’t thoughts that are the subject of thoughts, but something of a different order. To have a thought it is necessary for this other entity to exist; the thought is not a self-subsistent entity capable of independent existence. It depends on the existence of something outside of it. This contrasts with the position that the mind is merely a collection of free-floating states of consciousness, owing nothing essential to anything outside of those states. Only what is internal to thought is essential to its existence. Thus we cannot validly propound anything like the Cogito: from the existence of thoughts we can infer nothing but the existence of thoughts. Accordingly, thoughts could exist in a world consisting of nothing else as a matter of metaphysical possibility, whereas the Cogito asserts that thoughts require the existence of other things in order to exist. Thoughts require the existence of selves in order to have being at all, according to the Cogito, while they are conceived as ontologically autonomous by the contrasting position. This is why Descartes held that the Cogito provides leverage against the skeptic: it enables us to get beyond the mere existence of states of mind into a broader reality.
I have stated the Cogito in such a way as to bring out its analogy with externalism about mental states. For that doctrine also asserts that thoughts imply the existence of something beyond themselves—not the self, however, but the objects of thought. That is, it asserts that thoughts cannot exist in the absence of external objects that give them content: they draw upon objects distinct from themselves in order to be thoughts at all. There is thus an analogue to the Cogito: “I think, therefore the objects of my thought exist”; for example, “I think that London exists, therefore London exists”. The direct reference, Russell-Mill-Kripke-Kaplan, view of names implies a modified Cogito, now relating to the subject matter of thought not merely to its subject. Generalizing, thoughts are not self-subsistent autonomous entities that can go their merry way without reliance on anything else; they depend upon an ontological realm beyond themselves—what we are pleased to call the “external world”. Put differently, they are not merely subjective entities locked up in the mind but possess objective conditions of existence: no external world, no thought. The contrasting view is that thoughts only contingently have objective correlates; in themselves they are free-floating entities beholden to nothing outside their own boundaries. They exist entirely within the subject and could exist in the complete absence of an external world. As a corollary, their content cannot vary without some variation in their internal characteristics, notably their introspectively available nature: same inner appearance, same content. The externalist, by contrast, claims that content can vary while inner appearance remains the same (Twin Earth etc.). Here we have an analogue in the case of thoughts and their subjects: identity of thoughts implies identity of subject for the internalist about subjects, while the externalist maintains that the same thoughts could correspond to different selves thinking them. Consider psychological twins having exactly the same thoughts: one view says they must be the same self, since selves cannot transcend collections of thoughts, while the other view says that two distinct selves could have the same thoughts. In both cases the internalist insists that no reality beyond thoughts themselves need enter the picture—neither subjects nor objects—while the externalist holds that thoughts essentially involve an extra level of reality. The externalist sees thought as reaching out to selves and external objects, drawing them in so to speak, while the internalist sees thought as standing apart from everything else. The question at issue between them is whether the mind incorporates other regions of reality or whether it is sealed up in itself.
The point I am driving at is that externalism and the Cogito are in the same line of business seen from a broader perspective. The Cogito is a kind of externalism: it regards thought as incorporating an “external world” of selves—entities that are not reducible to thoughts and might even pre-exist them (depending on what exactly selves are). Similarly, the modern externalist holds that thoughts incorporate objects in the environment existing at some distance from the subject and generally pre-existing thought. The externalist thinks that thoughts have existential implications beyond themselves, just as Descartes thinks that thoughts imply the existence of selves distinct from themselves. And just as he argued that the Cogito thwarts the skeptic, so the modern externalist contends that externalism undermines radical skepticism: for both take thought to involve more than merely inner states whose nature is exhausted by introspection. Thoughts drive us in a direction beyond themselves, thus delivering us from the walled-in world of pure subjectivity—so at least it is supposed. Ironically, then, Descartes is the first externalist—the first to claim that thought is possible only because of things outside of thought. He is opposing the idealist notion that there could be nothing but ideas, i.e. states of consciousness, because ideas themselves require the existence of non-ideas—the selves that have them. Externalism merely pushes this point a stage further by contending that thought content is not independent of non-mental reality (particulars, natural kinds, properties, etc.). Thought involves the world at both ends so to speak. It embraces the world; it doesn’t shun the world.
In fact, according to the clear-headed externalist, thought doesn’t exactly “reach out to” or “embrace” or “incorporate” or “include” things beyond itself—though those locutions are immensely tempting—since it alreadyembodies the world in its inner architecture. A thought just is a subject apprehending objects; it doesn’t achieve this condition by starting from a different position. There is no reaching out to do, no implying or extending or grasping, since that is what a thought constitutively and originally is. The fact of thought contains subjects and objects as part of its inherent make-up, according to the externalist; it doesn’t need to stir itself into miraculous acts of inclusion, as if employing a magical lasso. It is not an essentially inner thing with remarkable powers of attraction; it is an outer thing from the start. That is, the Cogito and its externalist counterpart merely express what a thought is in its original nature. A thought is a subject-object complex for the committed externalist, where the subject and object are not themselves instances of something mental. If we are materialists, subject and object are both material things, so thought (in addition to its material realization in the brain) embeds both a material self (perhaps consisting of the body) and a material external world (tables and chairs, water, tigers, etc.). The Cogitoand its modern externalist counterpart situate the mind in a world of things whose nature is not inherently mental—selves and freestanding objects. Thus both oppose the idealist position that thoughts can exist in a world devoid of non-ideational being, as if they need neither a subject to possess them nor an object to be what they are about. There is no clean separation of mind and world of the kind the idealist presupposes.
 I won’t defend, or even much articulate, externalism here, but bear in mind that the doctrine comes in several varieties and that the external entities invoked can vary from material particulars to Platonic universals and numbers—anything that isn’t mental in nature. A relatively weak version of it says simply that thoughts concern properties and properties are non-mental attributes of external things: see my Mental Content (1989).
 Clearly I am sympathetic to externalism (of a suitably restricted kind), but in this paper I am suspending the question of its truth and merely exploring an analogy with the Cogito. It is perhaps surprising that such a famously Cartesian thesis should be so consonant with modern externalism, which prides itself on rejecting a (supposedly) Cartesian view of mind. Actually, I don’t think Descartes would have much objection to Twin Earth cases and the like.