Cogito for the External World
The traditional Cogito “I think, therefore I am” yields a moderate harvest of existential conclusions: the existence of a subject of thoughts (albeit momentary and etiolated) and the existence of propositions as the content of thoughts. We might compare this to a plant that is rooted in the earth and directed to the sun (earth as subject, sun as proposition). The plant exists, as it were, between the earth and the sun—as a thought exists between a subject and a proposition. It needs both, pointing down and pointing up; it “presupposes” earth and sun, as a thought presupposes subject and proposition. But does the thought (episode of thinking) bring with it any further existential implications? The plant also exists in a surrounding environment of physical objects and processes—does the thinking exist in a surrounding environment of objects and processes? If so, are these objects and processes logically implied by the thinking? In particular, is what we call the external world implied by the existence of thoughts? Is there a Cogito for the external world? The idea might seem incredible: how could a thought, an event of thinking, imply the existence of the external world? What about the evil demon and the brain in a vat? True, some philosophers have sought to deny that these scenarios are really possible—they have held that the content of thought requires the existence of suitable external objects in order to exist at all. Thus, we have “externalism” about the content of thought: what a person thinks is fixed by his or her actual environment and causal connections thereto. However, such views are not terribly plausible; in any case, I won’t discuss them. But I will discuss another line of argument that seeks to forge a logical link between mind and world—between the internal and the external. That is, I will defend the following Cogito: “I think, therefore the external world exists”; or, to parallel the traditional phraseology, “I think, therefore it is”—in Latin “Cogito, ergo est”. The “it” here is the physical world that exists outside the mind—the world of extended objects in space. We won’t get the entire external world as normally conceived, but we may get a part of it—and so derive from “I think” propositions concerning the existence of ordinary material objects. The first thing to notice is that thoughts (and other mental events) do in fact occur in an environment of physical objects, rather like plants; the question is whether this is an a priori necessary truth. Thus, thoughts occur in the vicinity of bodies and brains and interact with these objects, as a matter of actual fact; what we have to determine is whether this is built into the very identity of thoughts, a necessary condition of their existence (like subjects and propositions). Is it part of the nature of thoughts to be so situated? First, we can observe that thoughts have bodily effects: what you think affects what you do. And this is part of their identity: if you think it’s cold, you will dress warmly (given appropriate desires). If a thought did not have such consequences, it would not be thatthought. This is a familiar reflection. Of course, the thought might not actually have such consequences, but it is (as we say) disposed to have them (even if the brain of the thinker has been removed from the body, he still has a tendency to act in certain ways). The question is what this tells us about the thought itself: must it be somehow bodily? I think the answer is yes: the thought can have bodily effects only because it has a (partly) bodily nature. That is, the thought can only have a certain functional role vis-à-vis the body if it has a (partly) bodily nature. This nature will involve the brain; so, the thought has a cerebral nature of some sort. A materialist will say that its nature is wholly cerebral, but we need not commit ourselves to that, restricting ourselves to the weaker claim that it has a cerebral aspect. Accordingly, and in line with reasonable materialist sympathies, thoughts must have a physical nature of some sort (along with a possibly non-physical nature). But then we can deduce from the occurrence of a thought that something physical exists—that there exists something bodily that is built into the thought. This something is presumably complex and consists of elements that could exist without the thought existing (just like in the case of the brain), so we get the idea of a material world that can exist independently of anything mental. We get something like an atomic theory (bodies are made up of smaller physical components). Of course, we already know this to be true on empirical grounds, but now we see that it is deducible a priori from the very nature of thinking: if a thought exists, it must have a nature that allows it to affect the body (if there is one); but then it must have a bodily nature of its own; therefore, there must be something bodily in the world. Cogito, ergo est. I think, therefore there is an external world. Given that I know with certainty that an episode of thinking is occurring, and given that I know a priorithat this requires bodily existence of some sort, I can infer that a body exists in much the way I normally suppose. In fact, I can be certain of this—or as certain as I am that subjects and propositions exist given that thoughts do. I can infer a physical “environment” for acts of thinking much like the one I normally take for granted (centering on the brain). I can formulate a Cogito for the external world, or part of it. Lichtenberg is thus multiply wrong in his claim that nothing follows from “I think” apart from the existence of thoughts. And Descartes is wrong that the Cogito argument applies only to the self; in fact, the conclusion is more robust in the case of the external world than in the case of the self, since only a vanishingly thin notion of self can be derived from “I think”. Descartes, of course, cannot avail himself of this argument for the external world given his dualism—for him the mind has no physical nature at all—but this dualism is hardly something we would wish to hang onto. A reasonable dose of materialism is all we need to develop a modified Cogito that delivers substantive conclusions about the make-up of the world: we can now assert that the world is partly mental and partly physical. The existence of physical things follows from the existence of mental things; it isn’t merely a contingent add-on. Thus, thoughts are rooted in selves, point to propositions, and incorporate their physical environment (the brain). They imply the existence of something psychological (a self), something abstract (a proposition), and something physical (a body); they are not logically cut off from the rest of reality. They affirm more than just their own existence (a la Lichtenberg) but extend outwards into three domains. The Cogito has been underestimated.
 I won’t discuss how much of an inroad into skepticism this makes except to say, “Some but not much”.
 It has been overestimated in its original incarnation, as a proof of the existence of the self as commonsensically conceived; but it has been underestimated as a device for proving other existential propositions from apparently minimal resources. There is more in “I think” than meets the eye.