That the act of thinking guarantees the existence of the self is a fact that many philosophers take for granted. As Descartes famously put it “I think, therefore I am”, an assertion that has come to be known as the cogito. Certainly the cogito doesn’t seem like something that can be doubted, but if we are really to rely on it we should be certain that its truth can be divorced from Descartes’ philosophy. Here then I will attempt to reconstruct the cogito, as something that cannot be doubted, using a modern philosophical approach.
To defend the cogito we must first reduce it a simpler form that is easier to defend, which is an assertion that we must accept that doubts exist, since to doubt that assertion would be to prove it to be correct. However, even this form of the cogito may be attacked by a kind of radical skepticism. It is possible that what seems like a doubt to us is in actuality not a doubt, but only the appearance of one, and thus it is possible that we might consistently have something that seemed to us like a doubt about the statement “doubts exist”, since no real doubting is going on. To defeat this kind of radical skepticism we must define doubt in terms of presentations, and thus restate our initial claim as “there exist things that present themselves as doubts”. Of course the radical skeptic may now claim that we haven’t proved that real doubts exist, only things that seem like them. This might be a real problem if we believed that a conscious act was something that existed outside of consciousness and was made available to it. This is not, however, how we approach conscious acts, conscious acts simply are their appearances. Even if the conscious act itself is part of some larger unconscious activity, what defines the conscious act as a specific conscious act is how it presents itself to us, unconscious features are irrelevant. And so our rephrasing of the initial claim isn’t even a substantive change, simply a clarification.
Knowing that doubt exists we can generalize and assert that conscious acts exist, since doubts are a kind of conscious act (although we haven’t proven that anything besides doubt must exist), yielding “there exist things that present themselves as conscious acts”. From here, if we are to reach Descartes’ conclusion, we must somehow show that the self exists, and not just the conscious acts. There are basically three ways of understanding the “I” in “I think, therefore I exist”, as the real self, as the self that is constituted by the act of thinking, and as the first person perspective. Of these three it is obvious that the existence of conscious acts can’t prove that “real self” indeed exists. What I mean by real self is the properties of the mind / conscious mind that are really had by the individual, for example certain abilities, dispositions, memories, ect. It should be obvious that a deluded individual might construe his or herself as some other individual in their thoughts, for example as Napoleon. In their thoughts they might think of themselves as having Napoleon’s memories, Napoleon’s dispositions, ect, but in reality they have none of these things, their real self is not Napoleon. Thus it is not self-contradictory to deny that the self that one thinks that they are is their real self, although it is hard to see why anyone would do this. Similar considerations reveal that it is impossible to deny that the constituted self (the self as conceived of in conscious acts) exists, since to assert “I (or this person) is not my real self” is to constitute that self in the denial. The existence of the constituted self is, however, largely irrelevant, since in conscious acts we can constitute any number of such selves, and deny that they are us.
The really interesting question, then, is if we can show that the existence of a conscious act guarantees the existence of a first person perspective, and if some constituted self must exist as a result of this. The structure of consciousness, the fact that we talk of the conscious act as a presentation, certainly implies that the act is structured around a first person perspective. For the moment then I will take it as given that without a first person perspective nothing can be conscious (since otherwise who or what would it be an appearance to?), and thus to doubt that the first person perspective exists would itself be a conscious act structured around a first person perspective, confirming its existence. Why do I say structured around a first person perspective? Simply because a radical skeptic might insist that there is no real first person perspective, only an appearance of one.* So all we can really say is that the conscious act, the experience, seems as though it has a first person perspective, but given our discussion earlier about the contents of consciousness, this is all that really matters. And, as I alluded above, the existence of a first person viewpoint it itself a kind of minimal constituted self, since the first person perspective implies that there is someone who is having the current experience, even if it doesn’t necessarily give that self any other attributes.
Despite its age the cogito is still an important part of the philosophy of mind. Unlike Descartes, we no longer take it as founding all knowledge (or as proving the existence of god). However, even apart from Descartes’ projects, the cogito is still an important argument for the existence of the mind, since the cogito, as presented above, seems to guarantee that it is impossible to consistently doubt that something like consciousness and the first person perspective exist, a point that seems to escape eliminative materialists.
* In fact I agree, see here