What Did Descartes Mean by, "I Think Therefor I am?"
The Foundation of Knowledge
Those things which we perceive to surround us in the physical world, the ideas and propositions that we operate on as a matter of experience, and the possibilities we allow for in those dark recesses of the unknown all comprise the complex set of schemata's which, though often taken for granted, form a conduit allowing for relative predictability and homeostasis between sentient beings and their environment.
But in order to build this massive edifice of understanding we must first locate one cornerstone of undeniable absolute truth from which conjecture, extrapolation, and experiment can begin the arduous process of assembly.
Rene Descartes, the 17th Century Anatomist, Philosopher, and Mathematician first located and branded this cornerstone in his 1637 work. "Discourse On The Method," with the simple phrase, "Je pense, donc je suis."
Cogito Ergo Sum
He rendered this same proposition in Latin in 1644 in, "Principles of Philosophy," as, "Cogito ergo sum." From either language the English translation, "I think therefore I am," emerges.
By this simple proposition Descartes is asserting that the only thing we can be certain of is that we are conscious entities. All other things which we believe to be true may be illusion, deception, mistake, or imagination. We can doubt everything but the one proposition that because I am thinking my conscious mind, at the very least, exists.
If you find yourself thinking about this assertion and doubting it, you are actually further validating it's indisputable nature. In other words if you are doubting the proposition's veracity, you cannot doubt that you are in fact doubting and the process of doubt is an intentional act only available to a conscious mind.
"I, convinced that I was as open to error as any other, rejected as false all the reasonings I had hitherto taken for demonstrations...But immediately upon this I observed that, whilst I thus wished to think that all was false, it was absolutely necessary that I, who thus thought, should be somewhat; and as I observed that this truth, I think, therefore I am, was so certain and of such evidence that no ground of doubt, however extravagant, could be alleged by the skeptics capable of shaking it, I concluded that I might, without scruple, accept it as the first principle of the philosophy of which I was in search."
-Descartes "Discourse on The Method" (translated)
Principles of Philosophies
We find this thought somewhat refined and more clearly stated in 1644 with the publication of, "principles of Philosophy."
"While we thus reject all of which we can entertain the smallest doubt, and even imagine that it is false, we easily indeed suppose that there is neither God, nor sky, nor bodies, and that we ourselves even have neither hands nor feet, nor, finally, a body; but we cannot in the same way suppose that we are not while we doubt of the truth of these things; for there is a repugnance in conceiving that what thinks does not exist at the very time when it thinks. Accordingly, the knowledge, I think, therefore I am, is the first and most certain that occurs to one who philosophizes orderly."
-Descartes "Principles of Philosophy" (translated)
Quite simply put even the most preposterous things are possible thus making everything that we believe inherently doubtable. We may be artificially intelligent entities running on a historical simulation in the year 2500, I may have been created only a moment ago with the memory of having written this hub planted in my mind along with the rest of the past that I believe to be my own, but the one thing which I cannot doubt or repudiate is my own thought regarding this doubt. Thus the only thing which I know for certain is that I do think and therefor the conscious thought that constitutes the core of my existence must be real, at least in my subjective experience of it.
One criticism of the Cartesian proposition is that it necessitates a third contention to constitute a valid syllogism. From the argument, "I think therefor I exist," is missing the fulcrum of the syllogism, namely, "thinking things exist."
In a counter to this argument it has been contended that this proposition does not take a syllogistic form but rather that anything that has the ability to think exists self-evidently. Hintakka resolves this nicely when he explains that one cannot doubt that they exist because this results in the conclusion, "I do not exist, and I am wrong," which is an ontologically nonsensical.
Another critique is that Descartes deduces too much from the process of thought in supposing an individual agent. To divorce an independent agent from autonomous thought simplifies the proposition to, "thinking is occurring and thus thinking exists." To suppose that there is one particular thinker or another is too begin with the premise that Descartes endeavors to prove.
Nietzsche took issue with the presupposition of an intelligible, "I," or such an easily definable thing as, "thinking," and consequently whether anything could indeed know that it was thinking.
Bernard Williams constructed one of the best arguments in opposition to Descartes. This has to do with the problem of our own subjectivity being unverifiable objectively by a third party. Surely thinkers and thoughts themselves must be recognizable to an outside observer in order to become sensical and yet the ego cannot differentiate, as Descartes has already conceded, between the subjective and the objective. Conscious introspection cannot verify any third party validation that transcends the individual mind.
Most of the objections to Descartes assertion are at best philosophically pedantic and in some cases linguistically obtuse. Despite the above listed objections, the proposition differentiates itself from most of our beliefs about the world and thus renders it a special foundational truth that is self-evident at least within the paradigms of personal consciousness as we understand and use it.