Alex asked:
What is Descartes Methodological Doubt, and why does he insist/want to use it? What is the one “true” idea that is derived from this doubting? What is his proof for God (the trademark version ie the causal argument), and more importantly (ie specifically),what is the problem with it – why does his argument fail – please specify in detail this problem?
Answer by Craig Skinner
Descartes was tired of scholastic philosophy, viewing it as hairsplitting logic, nitpicking metaphysics, postulated occult powers, preoccupation with theological matters such as transsubstantiation and the holy trinity, and acceptance of Aristotle’s creaky cosmology and physics. On the other hand, he was enthusiastic about the new “mechanical philosophy” (physics), and thought such empirically-based science, coupled with mathematics, might yield more understanding and also control of nature to our benefit.
So he wanted to start from scratch and “build anew from the foundations… to establish… firm and permanent structure in the sciences”.
He says that many of his former beliefs were false or doubtful. So a new foundation had to be a belief that can be relied on as absolutely certain. How to arrive at such a belief? The first Meditation spells this out – his famous method of doubt.
He says he will doubt everything that can conceivably be doubted. This includes all beliefs based on the senses and all beliefs based on reason.
As regards the senses, we can doubt them because
(a) they sometimes deceive us, a commonplace observation.
(b) when I dream I think I am awake and doing things. So, at any time when I think I am awake, I might really be dreaming, and all the assumed external world an illusion.
(c) a malicious demon could put ideas in my mind suggesting an external world when no such thing exists.
As regards reason, he feels that although we think we know, say, 2+3=5 with certainty, again a malicious demon could trick us so that every time we add these numbers we make a mistake, thinking the sum is 5 when it isnt.
He concludes that the heavens, earth, colours, figures, sounds, all external things including his own body may be illusory.
What then is left as his foundational belief, his “one true idea” as you put it?
He tells us this in the second Meditation. He says that if he is doing all this doubting, he must be thinking, and so must exist. “I think therefore I am” (“cogito ergo sum”), as it is famously worded elsewhere in his writings.
Of course by itself this doesnt get him far. The world might consist of just one thinking thing, himself. To guarantee the rest of the world he needs the existence of the guarantor, God, a non-deceiving God at that, to be another certainty. He cant have this of course. First, to say that his clear and distinct idea of God can be relied on because the idea was implanted by God, is to beg the question. Secondly, no proof of God’s existence is sound.
But he has a go. He seems to think that God’s existence is readily evident to any diligent meditator, and that arguments are just heuristic devices to help the slower meditator to the almost self-evident truth that God’s existence is known by clear and distinct perception. So he doesnt set out his arguments formally with premises and conclusion. Also he uses a lot of scholastic terms. We meditators have to work hard to penetrate his arguments.
You ask about the causal argument (Meditation 3)
A fair reconstruction is as follows:
P1. I have the idea of a most perfect (omnipotent, eternal, infinite, benevolent) being (God).
P2. A cause must be at least as great (real) as its effect.
Conclusion: the idea of God cant come from (imperfect) me. Its cause must be God (or, impossibly, greater). God exists.
The argument is valid. To declare it unsound we therefore need to attack the premises. Both are vulnerable to attack.
Objections to P1:
(a) a finite mind cant have an idea of infinity (Gassendi’s view in 5th Objections). Descartes replied that we can. It’s our understanding that’s limited, not the the thing of which we have (limited) understanding. I agree.
(b) the meditator can claim not to have this idea. Descartes assumes we all have the same (God-given) innate ideas. We simply dont need to accept this. I dont.
Objections to P2:
P2 isnt easy to grasp. The discussion is in technical, scholastic terms. Two types of reality (being) are distinguished regarding ideas. The existence of an idea (its formal reality) is distinguished from its content (its objective reality). “Objective” refers to the object contained in the idea, rather like the modern use of “subjective” – it refers to the tree (say) in the mind not the tree in the garden. The notion of degrees of reality is then introduced. Ideas all have the the same degree of formal reality, all being mind states, but they differ in degrees of objective reality – lowest in a mode (modification of a substance eg colour), intermediate in a finite substance, highest in an infinite substance. P2 therefore expresses the Causal Principle that the degree of formal reality of the cause must be at least as great as the objective reality of the effect, leading to the conclusion that an idea whose content (objective reality) is infinite (such as Descartes’ idea of God) cant have its cause in a finite being (with less than infinite formal reality), such as me, only in God, so that God exists.
First objection: the idea of God can come from me – having some degree of perfection, I can posit higher and higher degrees indefinitely (Mersenne’s view in 2nd Objections). I agree.
Second objection: animals and plants (greater) derive from inanimate causes (lesser), Mersenne’s view in 2nd Objections. I agree.
Third objection: P2 is just an assertion. No evidence is given for it. I would generalize Mersenne’s objection to say that simple things plus simple rules can lead to complex things eg laws of nature plus simple initial conditions in our universe has yielded, atoms, compounds, galaxies, life and minds, so that the Causal Principle is false. To assume that a finite mind needs an infinite mind to cause it begs the question as to God’s existence.
In short, the first premise can simply be denied, the second premise is false and question begging.
In the end, Descartes doesnt get further than the cogito, so that, far from establishing a new foundation, his philosophical legacies are the idea of scepticism, and, arising from his idea that I am essentially a thinking thing, the notion of mind -body dualism. Scepticism is a commonplace now in philosophy, science and everyday thinking – we all accept that certainty is only to be found in logic and mathematics. Dualism displaced Aristotle’s substance/form view, but has proved sterile, and the superiority of Aristotle’s view is increasingly recognized.
Ask a Philosopher 
Since my answer is about Descartes’ scepticism, I take it you are asking about Hume’s scepticism, and whether it wrecks our pretensions to knowledge.
I think Hume’s mitigated scepticism is just healthy scepticism, and the only thing he wants to wreck is dogma.
Whereas Descartes’ methodological scepticism is a starting point for his argument, Hume’s is a conclusion he reaches after considering the frailty of our cognition, based as he thinks, on impressions and ideas. Hume concludes that reason is just the servant of our feelings, and we know much less than we think we do: we think the future will be like the past but this is a mere habit of the mind, attempts to prove it beg the question; we think we see causation but all we really see is constant conjunction, no actual connection between cause and effect; we think we have an enduring self, but all introspection shows is a bundle of perceptions; we think there is an external world etc.
But he doesnt then recommend that we live without believing anything (radical or Pyrronhic scepticism). He feels this is anyway impossible, nature having made us so that we cant help believing in an external world, causation, regularities of nature and selves. And it is reasonable to believe these things as probable truths.
He considers that all knowledge is either a relation of ideas (such as 2+2=4) or a matter of fact (such as Paris being the capital of France). So statements that are neither matters of logic nor matters of fact are meaningless. He therefore feels that talk of God’s essence and actions, immortality of the soul and other metaphysical ideas, is hot air, and he recommends that we “commit,,, to the flames” any book on such matters. All this provoked Kant, who admired Hume but wanted to write about metaphysics, to invent a third category of knowledge on which to hang metaphysics, namely synthetic a priori ie necessary truths known a priori but, unlike analytic truths (Hume’s relations of ideas), telling us something about the world. Whether there is such a thing as synthetic a priori knowledge is controversial (I doubt it) but I wont pursue it.
In conclusion, Hume’s scepticism is neither radical nor methodological, but sometimes described as mitigated. I call it just healthy scepticism. It’s built into science – hypotheses are held provisionally and may be amended or abandoned as more evidence accumulates. And it’s a good thing in ordinary life: religious fundamentalists and militant atheists alike could benefit from reflecting that their view might be wrong and their opponent’s right.
Hume’s epistemology has been characterized as a “wrecking ball.” Do you agree with this characterization? Why or why not?