At the start of the second meditation, Descartes seems to work his way to his first indubitable conclusion: "I am, I exist."
A question one might ask is: "how do you know you really exist?" After all, he might just be dreaming that he exists.
What might you think Descartes' response would be if someone asked him this?
A question one might ask is: "how do you know you really exist?" After all, > he might just be dreaming that he exists.
What might you think Descartes' response would be if someone asked him this?
In so asking, "How do you really know...?", one doubts. What doubts or who doubts was less important than that this doubting (thinking) operation occur(s), that it process whatever it doubt, whether a proposition ("I exist") or the reality of color (as opposed to wavelengths).
I think RodolfoAP's response is more canonical, but I would add that the cogito is more an operation (or sum of operations, like thinking, judging, doubting) than an I (or ego, subject, into which it froze in subsequent modern philosophy), that this "thing" for Descartes is as much an action as an agent (of its action).
"I indubitably exist as doubting, if I doubt I exist, therefore I exist, perhaps only as doubting, not howsoever I misconceived myself before."
Descartes' cogito ergo sum is not an ontological statement (I exist as an object of the world, which is certainly subject to your doubt) but moreover an epistemological one (I know about my existence). Descartes position was moreover I can be certain of my existence, but I cannot be certain of the existence of a world outside me.
In consequence, the Cartesian rule can be expressed as this: "I-subject think of me as I-object, therefore I-subject exist". Notice this is only one of hundreds of the cogito interpretations, which seems useful here.
Same idea, in simple words, using the later Berkeley's notion, esse est percipi ("to be is to be perceived", not his quote, apparently, but valid to express his position):
I perceive me [cogito], therefore I know I exist [sum], which is sufficient to satisfy the Berkeleian esse, and which has no relationship with existence in the world.
I thought Descartes explored this as much as possible, given his religious conditioning.
Descartes asks the reader to consider the possibility that an evil demon with "utmost power and cunning has employed all his energies to deceive me" could manipulate him into believing that he is thinking but easily dismisses this theory under God's infallible goodness.
The idea is that such an omnipotent evil demon would make us * believe* we are thinking, existing. As though our minds are at the whim of some absolute evil demon that can provide the sensation of existing or feeling or being, for nothing more than the mission of said evil demon.
Descartes doesn't refute this possibility but renders the outcome of such a scenario incompatible with reason or rationality, thereby effectively rendering any contemplation under such conditions a futile exercise unworthy of rational exploration.
Descartes concludes that thinking was enough of a premise to prove existence.
"Thus, I see that the certainty and truth of all my knowledge derived from one thing: my thought of the true God. Before I knew Him, I couldn't know anything else perfectly. But now I can plainly and certainly know innumerable things, not only about God and other mental beings but also about the nature of physical objects, insofar as it is the subject matter of pure mathematics."
Descartes's conditioning that God exists and that only good things can come from God while also supposing that knowledge of God's goodness is the foundation of all knowledge. God created me, and this mind for which I am contemplating my own existence. The act of thinking is enough to prove, only to ourselves, that we indeed exist. Meditations is Descartes's pursuit of pure rationality or ultimate truth.
It is in this pursuit Descartes determines he must prove his own existence before proving anything outside of himself.
