When Descartes investigated the implications of skepticism, i.e., that because we can cast doubt on any supposition, we can never be certain that our supposition is correct, he proposed that doubting ones own existence necessarily affirms it and, thus, of at least one thing a person can be certain beyond any doubt; that they exist. In order for an entity doubt anything at all, even that it exist, it must first exist to do so. Descartes proposal is immune to the skeptics doubt because the very practice of doubt confirms it to be true. Although I would not describe myself as a rationalist, as I agree with Kant’s interpretation of a priori knowledge, I do think it solves that problem of skepticism. In considering Descartes’ proposal, I find myself wondering if it is possible to contemplate ones own existence having never experienced the reality that existence must either define or be a constituent of.

I would argue that doubting, or any other form of thinking, is dependent up experience. When I think of what defines an experience, the first things that come to mind are the physical characteristics of the world that inspire my biological senses. Our biological sensations are of real, tangible qualities such as taste and smell. However, to experience such things is dependent on the passage of time. Just as active sensory perception is dependent upon time, so to is thinking. Try to imagine what it would feel like if time stopped completely. I doubt you would even notice because the beginning of a thought is not instantaneous with its end. Denying my claim would require that a thought both exist and does not exist simultaneously, thereby violating the law of non-contradiction. In fact, by that same reasoning, no beginning can occur simultaneously with and end.

The rationalist would assert that the knowledge of ones own existence can be reached by reason alone and is independent of our empirical experience. However, as I have attempted to show, it is required that we experience time if we are to apply reason at all.

So, like the empiricist and unlike the rationalist, I believe that knowledge requires us to experience our reality, even if our existence is the sum of all things logically knowable. Unlike the skeptic, though, I do not believe this dependency on experience casts doubt on certain absolutes. While my perception itself may be flawed, that I perceive at all can not be logically denied.

- Chalmer