I just started learning philosophy at school, so please forgive me if what I say sounds naive or has been disproved by philosophers. Basically, the teacher said that Descartes said "I think therefore I am" and meant by it that his existence comes from the fact that he can think. However, I didn't find this convincing and here is my argument. Saying I think therefore I am is clearly true: if I didn't exist I wouldn't be able to think. However, where I think it goes wrong is to say that this is the base for my conciousness. Because saying I think therefore I am must mean that "I think" is true (otherwise we can't derive "therefore I am" from it). So it is necessary that "I am certain that I think", and this in turn necessarily implies "I am certain". But who is certain? Of course, it is "I", so this means that certainty comes from "I" not from "I think". I am not sure if Descartes considered the two to be the same, but "I" certainly seems to refer to conciousness by itself whereas "I think" seems to denote conciousness plus an act of thought.
I think the cogito relates to Descartes' clear and distinction perception of himself thinking, which necessarily entails clear and distinction perception of himself as a living being. His argument moves from clear and distinct perception of himself thinking to clear and distinct perception of himself-as-being - the certainty is supposed to derive not from thinking itself, but rather clear and distinct perception of himself thinking, which perception is in general shown to be impossible unless one always and at the same time has clear and distinct perception of oneself-as-being.
Descarted did not say "I am therefore I think". Nor did he ever imply that.
He said "I think therefore I am", meaning that his capacity to think is a sufficient reason for Descartes to conclude that Descartes exists.
The main flaw in that argument, is that it's rather meaningless without first defining what it means to exist. If the universe is just a giant hologram, as scientists today are claiming, what does it mean to "exist" in that context? And can we draw the conclusion that one exists if one is able to think, without understanding first what it means to "exist"? It seems to me that the more we learn about the nature of the universe, the harder it becomes to define what it means to "exist".
Here, I believe, is where Descartes fall short!