Descartes later cheated on himself by using entirely separate arguments, not deduced from the Cogito, to prove that God did after all exist and that He was not a Great Deceiver who would deceive Descartes about what came to him via his senses. So Descartes could now accept what his senses told him, and discard Cogito ergo Sum entirely!
His arguments that God existed were painfully feeble. But they gave him the opportunity to skip over ‘I think, therefore I am’ as the only thing he could be certain of, and to now believe in his God-given senses instead. Brilliant! — he could now believe that he did have a body with arms and legs, that the outside world did exist, and that he hadn’t really needed his Methodological Scepticism and Cogito ergo Sum in the first place (from here ).
And what were these proofs of God’s existence? One of them was pretty much Anselm’s of 600 years earlier (from here , here, and here ) which went as follows: ‘God exists in my mind as the Greatest Being. Existing in reality must be a quality of the Greatest Being. Therefore God must exist in reality.’ This quality of argument is dignified by the title ‘Ontological Proof of the Existence of God’. I can only think it is called ‘ontological’ because it goes back to Parmenides’ Ontology which says, ‘If you can think it, it is, it exists in reality!(from here).
It is amazing that even Anselm took it seriously. Parmenides’ idea was potty in 500 BC, Anselm’s in 1078, and Descartes’ in 1637, but subsequent philosophers have still thought it worthwhile to explain what was wrong with it. It is said that Descartes wished to deduce that God existed because he was a believer, and because he needed to avoid Church persecution.
Descartes, I think, got back to the world of the senses as soon as he could, from a world of complete insanity.
Descartes, it seems, used a numberof other arguments too for the existence of God. There was one similar to Aristotle’s that there has to be a First Cause for everything that exists or that happens. There was also an argument, similar perhaps to another of Aristotle’s, but I haven’t quite got hold of it. Also a ‘Causal Argument’ — that if one has an idea of something, then there must be a cause of this idea greater than or at least as real as the idea itself. This latter argument may come from Augustine’s neo-Platonism, although it seems to me similar to Anselm’s. (This paragraph is derived from a post by philosopher Akomolafe Akinola Mohammed that is no longer on-line .)
Some philosophers, who probably all believe in the senses, have criticized Descartes’ particular reasons for coming to believe in them, because these reasons included a belief in God. And this latter belief rested on a priori metaphysical arguments such as Anselm’s ontological one. It didn’t rest on a posteriori sensory evidence because Descartes could only trust his senses after he had proved God! These later philosophers, I presume, were of the 20th century kind who only respected a posteriori sensory evidence and not metaphysical argument.
Some philosophers also point out that Descartes’ proof of God’s existence contained ideas that Descartes assumed to be ‘clear and distinct’ which already assumed the existence of God and the reliability of the God-given senses ; see here.
I suppose Descartes could have given naturalistic evidence for God’s existence – from the orderliness of Nature – rather than the potty metaphysical argument of Anselm.
[Here is something of a side-issue: At some stage Descartes did come to believe that ‘My clear and distinct impressions must be true’. I had thought he had derived this from the Cogito while he still believed in it, and that his ‘clear and distinct impressions’ were logically analytic statements or mathematical statements, that are necessarily true because of the words or numbers they consist of in the first place, such as that old chestnut ‘All bachelors are unmarried’ (which seems to take us forward to Kant).
But no, that last paragraph is probably an entire misunderstanding of mine. It seems rather that what Descartes meant was that clear and distinct perceptions by the senses can’t be wrong, and he came to believe this after proving to himself that God existed and that the God-given senses can’t be wrong.]
Philosophers are still taking him seriously, and saying that his thinking had important consequences for later thought! He has gone down in the history of Western philosophy for ‘methodological scepticism’, ‘epistemological idealism’, and ‘Cartesian Dualism’; and as the ‘the first modern philosopher’, and who ‘provided a foundation for the natural sciences’. My mind boggles.
It seems that one consequence of Descartes was that thinkers could now investigate the world (by deduction or by sensory perception) without fear of the Church. This was apparently because Descartes proved that God existed and that he wasn’t a Great Deceiver, and that one could trust one’s God-given senses to perceive the world. So it became theologically permissible for thinkers to investigate the world by thought, because they had already accepted God’s existence (from here). (Was it previously frowned on to investigate the world, because it implied an atheistic disbelief in the Bible and the Church’s ‘s truths about the world ?)
So the Church could now be tolerant of philosophers’ thinking about the world. But it seems that Descartes also helped Philosophy become an enemy of Religion. Here is an excerpt from George Heart’s Christianity: Dogmatic Faith and Gnostic Vivifying Knowledge that I got from a site I now can’t trace, possibly available here, and that I edited myself as follows: ‘Philosophy was independent when it began, but then Christianity appropriated it for its defence of the faith, showing faith to be in accord with Reason. Reason/Philosophy was seen as a helper of religion. This came to a head with Scholasticism. Descartes broke philosophy free from religion. That is his importance as Father of Modern Philosophy. Reason on its own could now understand God, universe and man. It had its own validity apart from divine inspiration. Philosophy became an enemy of religion.’
So did Philosophy become independent of Religion, partly at least through Descartes showing that Reason could prove the existence of God without help from the Bible? But Descartes’ proofs by Reason were ludicrously feeble. And the God he proved was a deistic one, not specifically the God of Bible or Church. The mind boggles.
Descartes also contributed original work to mathematics and to science (from here and here, and more on him here.) He invented analytic geometry, provided the basis for calculus and ‘thus for much of modern mathematics’, and did seminal work in optics. I say this to show what kind of mind he had.
But, against my disdaining of philosophers as having logicking, mathematicking and scientificking mentalities, he said this about himself in his Discourse on the Method: that, early in his life, he ‘abandoned the study of letters. Resolving to seek no knowledge other than that which could be found in myself or….in the great book of the world, I spent the rest of my youth travelling, visiting courts and armies, mixing with people of diverse temperaments and ranks, gathering various experiences, testing myself in the situations which fortune offered me, and at all times reflecting upon whatever came my way so as to derive some profit from it .’ He was even a military mercenary.
At about the time of Descartes, John Donne and Andrew Marvell were writing too. And, even better, compare all this philosophizing from Parmenides to Descartes, to what a country rector’s daughter wrote in a novel in about 1815: ‘A very narrow income has a tendency to contract the mind and sour the temper’. This is a popular chestnut that I had ready to hand, but these precise words represent more of Wisdom than Philosophy ever did. Compared to these writers, Descartes is a schoolboy retarded. I think ”sensibility to human life’ comes closest so far, for me, in understanding what philosophers’ minds lack, despite Descartes going out to get experience of human life.
(I am surprised that it was still Philosophy and not Science that was expected to reveal Truth in 1637 AD, but perhaps scientific method had only recently started.)