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After his ever so famous "cogito ergo sum", Rene Descartes' second (deep?) thought was something like "God exists" (according to my literature). I think he brought this up mainly due to historical reasons or personal belief, but to me (although I believe in something) that's far less obvious than how he started out.

Why didn't he at least continue with something like "there exist impressions, that fed my mind, such that I can doubt their existence, because I think they are created by putative evil demons" or "there exist senses, that fed my mind with data, be it right or wrong"?

To me, these two expressions are as clear as the first one. Am I right and are there other simple truths?

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    Not at all. That conclusion is based on his ontological arguments, not cogito. Re-read the Meditations (Meditation 5 in particular). This SEP article should help. – Hunan Rostomyan Mar 24 '14 at 22:12
  • Not at all what? You mean the conclusion that God exists is not based on cogito? I agree on that. But what about "there exist senses that fed my mind"? – draks ... Mar 24 '14 at 22:27
  • The 'continuation' you're offering is something he began with, namely that sense impressions cannot be trusted (given the possibility of the evil demon). Cogito is something that follows that thought. – Hunan Rostomyan Mar 24 '14 at 22:36
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    Well, in that case: what is your question? – Hunan Rostomyan Mar 24 '14 at 22:39
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    That there is a sensation is indubitable, actually. While the existence of the sensation is indubitable, the possibility of the evil demon makes it impossible to say whether the sensation has the appropriate kind of causal relations to the world. Concretely, if you look at object x and see it to be a red apple, it is indubitable that you're experiencing redness!, but it is dubitable whether x is actually a red apple (because it's possible that the evil demon is intercepting the light emanating from a green apple and messing with the frequencies before it gets to you). – Hunan Rostomyan Mar 24 '14 at 23:31
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The path Descartes followed from the "discovery" of the "first truth" : cogito, ergo sum to the existence of the self, God and the external world was a complex one, made of all the six Meditations, with a lot of subtle arguments :

The cogito raises numerous philosophical questions and has generated an enormous literature.

You must follow this path at least trough the relevant SEP entries : Descartes' Ontological Argument and Descartes' Epistemology :

Descartes' reference to an “I”, in the “I think”, is not intended to presuppose the existence of a substantial self. In the very next sentence following the initial statement of the cogito, the meditator says: “But I do not yet have a sufficient understanding of what this ‘I’ is, that now necessarily exists” (Med. 2, AT 7:25). The cogito purports to yield certainty that I exist insofar as I am a thinking thing, whatever that turns out to be.

In the final analysis, Descartes thinks he shows that the occurrence of thought depends (ontologically) on the existence of a substantial self — to wit, on the existence of an infinite substance, namely God (cf. Med. 3, AT 7:48ff). But Descartes denies that an acceptance of these ontological matters is epistemically prior to the cogito: [...].

If the cogito does not presuppose a substantial self, what then is the epistemic basis for injecting the “I” into the “I think”? Some critics have complained that, in referring to the “I”, Descartes begs the question by presupposing what he means to establish in the “I exist.” Among the critics, Bertrand Russell objects that “the word ‘I’ is really illegitimate”; that Descartes should have, instead, stated “his ultimate premiss in the form ‘there are thoughts’.” Russell adds that “the word ‘I’ is grammatically convenient, but does not describe a datum.” Accordingly, “there is pain” and “I am in pain” have different contents, and Descartes is entitled only to the former.

But note that :

As the canonical formulation has it, I think therefore I am. (Latin: cogito ergo sum; French: je pense, donc je suis.) This formulation does not expressly arise in the Meditations.

  • Thank you, that was really quite interesting to read. I added the german translation. What about my question, concerning my own thoughts? – draks ... Mar 25 '14 at 22:00
  • @draks... - the german translation was not included because the paragraph is extracted from teh SEP entry and it includes only the latin and the french versions (I think because they are extracted form Descrates' later work Principia Philosophiae (1644), published in Latin and translated into French (1647). – Mauro ALLEGRANZA Mar 26 '14 at 8:25
  • I think that, basically, the aim od D was not to prove that "I" exists (and I feel, perceive, and so on ...). This is the "easy task". He want to prove that the "external" world exists: that what I feel and perceive is not an illusion but the effect of somthing independent from myself that act on me and that my "perception" of this external reality is (at least in part) "reliable". In order to achieve this goal, he needs the existence of a (non-deceiving) God. – Mauro ALLEGRANZA Mar 26 '14 at 9:50
  • If you mind, please remove the german translation. So you say that my own thoughts are kind of trivial? But still they are true, right? – draks ... Mar 26 '14 at 11:22
  • @draks... - I'm not saying that. From the fact that I think and I feel, what I can infer ? According to D, I'm not licensed to infer directly that there is something real outside me that act on my sense and that my sensations are faithful witnesses of that "something". Only through the certainty (or belief) in the existence of a non-deceiving God, I can assert this conclusion. – Mauro ALLEGRANZA Mar 26 '14 at 11:52

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