1)What is the certainty that Descartes discovers in the Second Meditation?
2)What does Descartes go on to attempt to prove in the Third Meditation and how is this proof related to what was established as certain in the Second Meditation?
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The question is formulated in such a way that it cannot be answered: for , precisely, the discovery made in Meditation II is not Descartes's discovery , but the discovery the " I " ( the meditating Ego ) makes concerning himself.
There are 2 discoveries.
(A) First, the Ego discovers what makes certain the proposition " I am ". : the discovery of subjectivity.
Before engaging in the meditation process, the Ego thought he was René Descartes, an individual being belonging to a species of living beings called human beings inhabiting a planet called Earth rotating around a sun, un a physical universe, etc...
But this Ego says to himself
(1) If I were essentially the same being as Rene Descartes, a human being, with a biological life, living on earth, etc then, in case René Descartes' body did not exist, in case René Descartes memories were complete illusions, in case the earth, the sun, the physical universe were complete illusions caused by a " malin génie" , then I would not exist myself!
(2) But, that's totally false: it is not true that if all those things were illusions, I would not exist myself! to the contrary, it would prove that I exist ! ( I couldn't be subject to illusions without thinking, and therefore without being! furthermore this hypothetic " malin génie" could not deceive me if I did not exist! )
(3) So what makes the proposition "I am " an absolute certainty is not the proposition " René Descartes belongs to the world" or " Descartes lives, breathes, I eats, walks" etc. The certainty of this proposition " I am" rests on an interior ground, namely it rests on the ground " I think"! " No exterior "authority" can contradict me on this point , even the authority of God, even this " malin génie", this is a certainty that comes totally from within : " So that after having reflected well and carefully examined all things, we must come to the definite conclusion that this proposition: I am, I exist, is necessarily true each time that I pronounce it, or that I mentally conceive it."
Remark - This discovery is sometimes names the discovery of the " subject" or of " **subjectivity**" : the I discovers himself as ( at least conceptually) different from the human being he thought he xas identical to; he discovers that the absolute ground of truth and certainty is within himself, that this source of truth is consciousness itself ( awareness of one's thought).
(B) Second, the Ego discovers his essence, not only that he is or exists as a subject, but what he is.
Here, the Ego proceeds by elimination from his old belief " I am a man", " I am an animal that is also rational" ( Aristotle's definition of man) He substracts from that belief everything that is not absolutely certain ( body, vegetative soul, senstitive soul) and arrives at the conclusion that the only thing he can attribute to himself with an absolute certainty is thought: " I exist, that is certain. But how often? Just when I think; for it might possibly be the case if I ceased entirely to think, that I should likewise cease altogether to exist. I do not now admit nything which is not necessarily true: to speak accurately I am not more than a thing which thinks, that is to say a mind or a soul, or an understanding, or a reason, which are terms whose significance was formerly unknown to me. I am, however, a real thing and really exist; but what thing? I have answered: a thing which thinks."
The discovery can be best understood when comparing the new answer to the question "what am I?" to the " old one" ( Aristotle). Aristotle : What am I? I am a man, that is **a living body that HAS a
soul (intellectual soul)**. My soul is not a substance ( a complete thing)
it is a part of myself, namely the form of my body).
The meditating Ego in Descartes : What am I? **I AM a thinking
soul ( " ratio" , intellectus" , " mens" ) that HAS a body**.
( I am a substance, a thinking substance called "soul" - intellective soul -
and my body is another substance - an extended substance: these two
substances are united in a whole called " the man" , but
theoretically they could perfectily exist without one another).
Final remark :
Descartes is not an idealist, his final goal is to establish ( in opposition to medieval philosophy) the TRUE HIERARCHY OF CERTAINTIES
certainty of myself > certainty of God > certainty of the physical universe
One has to read the 6 Meditations to fully understand Descartes' efffective intentions.