Descartes later did a volte-face by jettisoning his Cogito ergo sum entirely, and now accepting what his senses tell him!
He did this by using entirely separate arguments, not deduced from the Cogito at all, 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!
Descartes, I think, got back to the world of the senses as soon as he could, from a world of complete insanity (of believing he could only get to truths if he started deducing from Cogito ergo sum).
His arguments that God existed were painfully feeble. But they gave him the opportunity to now believe in the senses that God had given him. 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 ). Brilliant!
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….’(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, it seems, used a number of other arguments too for the existence of God. There was one similar to Aristotle’s that there has to be a First Cause for anything that exists or occurs. (There was also another 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’ reasons for believing in these proofs of God’s existence, because these reasons included a belief in God. So, they were cicular. And they also rested on a priori metaphysical arguments such as Anselm’s ontological one; they 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 reliability of the God-given senses; see here. So he proved God’s existence by assuming God’s existence.
I suppose Descartes could have given ‘naturalistic’ evidence for God’s existence – from the orderliness that Nature displays all around us, which Somebody must have organized — which would not have been a circular argument. But he seems not to have done.
Here is something of a side-issue, already mentioned in Descartes1 in this site: 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’ .
But no, 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 for people who have taken Philosophy seriously, 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 the senses that God had given us to perceive the world. So it became theologically permissible for thinkers to investigate the world by thought or by scientific method using their senses, because they had already accepted God’s existence (from here).
(Was it previously frowned on to investigate the world, because it implied disbelief in the Bible’s truths about the world?)
So Religion could now be tolerant of philosophers doing their 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.’
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.
To understand what kind of mind Descartes had, one notes that he 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. So he was a towering figure in maths and science, let alone Philosophy.
Yet also, 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.
So, according to him, he did in fact have wide and deep, adventurous and dangerous experience.
At about the time of Descartes, John Donne and Andrew Marvell were writing too. And, even better, here is an old chestnut from 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’. 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.)