“So after considering everything very thoroughly, I must finally conclude that this proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind.” 

– Rene Descartes, Meditations on First Philosophy

How do you know you exist? To what extent can you doubt you exist? Rene Descartes compels us to explore the idea of doubt in a way that lets us question the knowledge of our existence. In his meditations, he introduces us to a certain kind of inquiry where he has us doubt everything that we could possibly doubt—whether it’s the validity of our senses or the already preconceived knowledge we have about our existence. Descartes’ starting point of inquiry is to see whether there’s anything he cannot doubt. And the one thing that he cannot doubt and know for certain is, the fact that there is thinking and doubting occurring. For Descartes, the subject of the thinking and doubting becomes the main thing being inquired about—an I that thinks and doubts, hence leading to one of the most groundbreaking philosophical statements, “I think therefore I am.” (cogito, ergo sum).

In Meditations, Descartes posits doubt as a potent philosophical tool—a way of getting to the utmost certain truth that we could possibly arrive to. He takes us on a philosophical train of doubting everything we know in order to eventually arrive at a conclusion that cannot be doubted. By having us doubt our senses, perceptions, and beliefs, that “the sky, the air, the earth, colours, figures, sounds, and all external things, are nothing better than the illusions of dreams”, Descartes attempts to demonstrate that it is possible to doubt everything we believe to be true and still be left with something we cannot doubt: that there is doubting happening. By creating a hypothetical scenario in which everything we perceive could be an illusion, he attempts to establish the existence of doubt and a subject of that action. The very fact that there is something there to doubt means there must be something that is doubting.  

How far can doubt take us? Senses, perceptions, and beliefs may be prone to doubt, but can one also doubt the notion of a subject that is doing the doubting? Just because there is doubt, does it necessarily imply the existence of a thinker or a subject of the doubting?  The inquiry doesn’t have to stop at the certainty of I; it can lead to doubting the very notion of one’s existence as a subject and the possibility that it could be mistaken about its existence. Descartes asks, “But what then am I? A thing that thinks. What is that? A thing that doubts, understands, affirms, denies, is willing, is unwilling, and imagines and has sensory perceptions.”    

 By defining the self as a “thing that thinks”, Descartes suggests that the essence of the self lies in the ability to think, reason, and perceive. He also lists other mental faculties such as doubting, understanding, affirming, denying, willing, and unwilling, which he believes are all necessary attributes of the self. In other words, he believes that there must be something that is doing the thinking, and that this thing must be a subject, or what we call “I” when referring to ourselves.   

We thus arrive at the same question: Just because there is thought, does this necessitate a subject that is doing the thinking? Perhaps the act of thinking itself could arise without a thinker, and the sense of there being a self who is thinking could also just be another thought or mental construct. The action of thinking may simply be a natural phenomenon, like the weather or the movement of the stars. 

From a linguistic approach, we can also doubt the existence of  “I” as a “thinking thing” to be a mere linguistic construct, rather than a real, existing entity. In other words, a product of language and social conditioning. When we say “I,” we are referring to a collection of experiences, memories, and thoughts that we have accumulated over time. In other words, we are using language as a tool to create a sense of identity and meaning around experiences, memories, and thoughts which constantly change and evolve. We use language to construct these mental images of ourselves, which we then believe to be our true self, thus creating the notion of a subject. 
But even if we doubt the existence of a thinking subject, we cannot doubt the fact that there is thinking occurring. How would this come about, if not for a state of prior existence that is responsible for generating these thoughts? In order to ascribe meaning to our very existence that is shaped by such thoughts, we have to assume there is a thinking being, and that the being exists because it is thinking.