“I think, therefore I am”
A philosophical Latin statement used by René Descartes, which became a fundamental element of Western philosophy. The simple meaning of the phrase is that if someone wonders whether or not he exists, that is, in and of itself, proof that he does exist (because, at the very least, there is an “I” who does the thinking). It forms the bedrock for all knowledge, because, while all things can be questioned as to whether they are from the realm of reality or from some figment of imagination (a dream, influence of a demon, etc.), the very act of doubting one’s own existence serves as proof of the reality of one’s own existence.
A common mistake is that people take the statement as proof that they, as a human person, exist. However, it is a severely limited conclusion that does nothing to prove that one’s own body exists, let alone anything else that is perceived in the physical universe. It only proves that one’s mind exists (that part of an individual that observes oneself doing the doubting). It does not rule out other possibilities, such as waking up to find oneself to be a butterfly who had dreamed of having lived a human life.
There have been a number of criticisms of the argument. One concerns the nature of the step from “I am thinking” to “I exist.” The contention is that this is a syllogistic inference, for it appears to require the extra premise: “Whatever has the property of thinking, exists”, a premise Descartes did not justify. In fact, he conceded that there would indeed be an extra premise needed, but denied that the cogito is a syllogism (an instance of a form of reasoning in which a conclusion is drawn from two given or assumed propositions (premises); a common or middle term is present in the two premises but not in the conclusion, which may be invalid (e.g. all dogs are animals; all animals have four legs; therefore all dogs have four legs).
To argue that the cogito is not a syllogism, one may call it self-evident that “Whatever has the property of thinking, exists”. In plain English, it seems incoherent to actually doubt that one exists and is doubting. Strict skeptics maintain that only the property of ‘thinking’ is indubitably a property of the meditator (presumably, they imagine it possible that a thing thinks but does not exist).