My commentary is in blue, excerpts in black.

What's with all this certainty business?
And so what if I exist?

From the Third Meditation (Vol.1, p. 24)

I am certain that I am a thinking thing. Do I not therefore also know what is required for my being certain about anything?

This is, to be sure, a bold claim! Does it follow from the fact that I know one thing for certain, that I also know what is required for certainty about any claim?! Well that's what it looks like he's getting at, and it's hard to interpret this as meaning anything else. Look at what he goes on to conclude:

In this first item of knowledge there is simply a clear and distinct perception of what I am asserting; this would not be enough to make me certain of the truth of the matter if it could ever turn out that something which I perceived with such clarity and distinctness was false. So I now seem to be able to lay it down as a general rule that whatever I perceive very clearly and distinctly is true. (my emphasis)

Does this seem like a premature or unwarranted conclusion? I think it is, but I also think that it's mainly meant to foreshadow what is to come. For it is here, in the third meditation, that Descartes proceeds to reason toward the conclusion that there is a god, and that he is not a deceiver. But there is another aspect to this bold claim, and I think it has everything to do with the naive meditator.
  See, because the meditator is likely to be a reader, call him John, who has "never philosophized correctly" [1], Descartes wants to show John what it's like to have a clear and distinct perception. That's probably the main purpose of the Cogito [2].

But what of God's existence?

"[I]f the mere fact that I can produce from my thought the idea of something entails that everything which I clearly and distinctly perceive to belong to that thing really does belong to it, is not this a possible basis for another argument to prove the existence of God?" (Vol.1, p.45 [5th meditation]).

The "something" in this passage is an abstract something, like a number or geometrical shape. These are not extended (infinitely divisible) things. We cannot be wrong about an idea of a triangle that has no (and has never had a) corresponding physical object. These kinds of ideas, of abstract objects and truths of mathematics, cannot have material falsity or formal falsity. Such an idea cannot be materially false, for in order to be false in this sense, it would have to misrepresent its object. But how do we misrepresent a triangle or mathematical theorem?
  Well, we can misrepresent a theorem as true when in fact it is false. But this is not the kind of idea Descartes is talking about here. For if the cognizer perceives clearly and distinctly that x belongs to an idea, then the cognizer can't be mistaken about that. To see why this is so, consider a case in which x is an obvious mathematical truth (in a Euclidian geometrical framework):

    x = {if one angle of the triangle I am thinking of measures 90 degrees, and the lines forming it on both axes are equal in length, then the other two angles must each measure 45 degrees}

Here the properties in questions are, among other things, the specified measurements of the angles.

"[F]rom the fact that I cannot think of God except as existing, it follows that existence is inseparable from God, and hence that he really exists" (Vol.1, p.46 [5th meditation]).

Well then, what does this thinking thing have to do with whether God exists? For one thing, existence is contained in the idea of god because absolute perfection is, too. So we can't have the idea of god without realizing, with absolute clarity and distinctness, that he exists! But not only does he exist, he exist as an entity external to us (we don't create him in our minds), and as an entity with actual existence (it's not that he once existed and now doesn't, or that he will exist but actually doesn't).
  Does this mean that we create, in our minds somehow, the idea of a thing that exists externally and actually (this is s alightly different point from the one above). Maybe we create the idea with all the properties mentioned in the previous paragraph. NOT SO. It's not our idea that brings it about that god exists in the way here specified: it is the real entity -- god -- that "makes it so, or imposes necessity on" not just my thought of him, but on everything in which necessity resides. "[I]t is the necessity of...the existence of God...which determines my thinking in this respect." "For I am not free to think of God without existence (that is, a supremely perfect being without supreme perfection) as I am free to imagine a horse with or without wings" (Vol.1, p.46 [5th meditation]).
  It seems more than a little compelling to conclude that for Descartes, "God exists" is analytic. The reason: the concept or idea of God contains the concept of supreme perfection, and the concept of supreme perfection contains that of actual and external existence. So however we interpret the "exists" in "God exists", I think, it comes out analytic -- necessarily true by virtue of concept containment.

[1]
In replying to Bourdin's objection in the seventh set of objections with replies, Descartes explains to his critic what he was doing in the Meditations, and why he wanted to begin by doubting everything that could be doubted. Interestingly, here Descartes says that "those who have never philosophized correctly have various opinions in their minds which they have begun to store up since childhood, and which they therefore have reason to believe may in many cases be false" (Vol.2, p. 324 [7th set of objections and replies]). Of course, this looks like it could describe everyone of us! For we have all been storing opinions since childhood.
  But if all Descartes means here is that we were all once naive children who stored beliefs prematurely, why would he refer to those who have never philosophized correctly? I think this is a compelling piece of evidence to the effect that the style of the Meditations was somewhat rhetorical because he wanted to lure the reader into philosophizing correctly; he realized that for many readers, this would be a first. So could it be that the Meditations was written specifically for such people? Maybe this passage indicates that they were.

[2]: The Cogito
I exist: but what am I? A thing that thinks. This is the most certain conclusion that can be drawn, according to the standard interpretation. One thing is certain on any reading, Descartes thinks this cannot, under any circumstances, be doubted. What can be more certain than this bit of knowledge?!

Material falsity
Material falsity "occurs in ideas, when they represent non-things as things" (Vol.1, p.30 [3rd meditation]). So material truth and material falsity corresponds to accurate representation and misrepresentation, respectively.

Formal falsity (or "falsity in the strict sense")
Formal falsity "can occur only in judgments" (Vol.1, p.30 [3rd meditation]). So this seems to be the kind of falsity we typically attribute to declarative propositions, like 'there is a candle before me'.

• reason used right doesn't err
• 'method': reliable easy to apply rules, follow them and can't go wrong
• without method, pursuit of knowledge more harmful than profitable

...to do as he saw fit...the method used...

P.419

spontaneous assent
REFERENCE

reply to Bourdin
7th set obj w/ replies

Bourdin: god idea is old news & c&d-->true, too

Letter to Clerselier
In Appendix to 5th Obj/Replies

author of Counter Objections (gassendi)

Letter to Clerselier
In Appendix to 5th Obj/Replies

author of Counter Objections (gassendi)

reply to Gassendi
5th set replies

Gassendi: Can't just put forth the C&D-->true rule!

Descartes: People who die 4 atheism aren't perceiving C&D'ly

Rule is justified conclusion because product of methodic doubt

To Gassendi
5th set replies

Gassendi: You didn't need all the doubt; could have gotten cogito from any action

...you continue to employ rhetorical tricks

P.243

2nd set of replies
to Mersennes
  in definitions

what to say???

idea definition

P.113

2nd set of replies
to Mersennes

in 7th set to Bourdin:
C&D-->true

What is it to us...

P.103

2nd set of replies
to Mersennes

Mersennes: not ready for cogito because no god yet

Rule#3: Intuition vs. deduction
Intuition simpler than deduction

Thirdly...God exists...deduction...

P.100

2nd set of replies
to Mersennes

Discourse Part1; doing somehting new; making the knowledge he hoped to find

Precooked material

P.94

Med.6

See contra: Med.1

Med.1

See Med.6

Med.5

analyticity: ontological argument

3 lines bind a triangle

Med.5

analyticity: ontological argument

3 lines bind a triangle

The mere fact that I can produce from my thought...really belongs to it...

Med.4

the will is not the source of error

broadsheet: assigns act of judging to will, not intellect

principles Part1 #42:
cause of error = too eager to assent, lack perception of x but still assent

...inquire into the nature of my errors...will...intellect

Med.3

ontological argument is an instance of this

reality gotten from more real cause

formal = literal
objective = representational

...there is another way of investigating...formal & objective reality

Med.3

2nd set of replies
to Mersennes: what is it to us...

in 7th set to Bourdin:
C&D-->true

general rule...is true

Med.1

Contra Med.6:
Big difference between dream and waking

brilliant piece of reasoning!

Parrots and Monkeys
  The Objection of Objections

Descartes addresses those who claim that he has not been able to escape skepticism regarding the existence of an external world. He goes on to conclude that if the conclusion of this objection is right, then we are all nothing more than parrots and monkeys.
  This passage kept me guessing for a while, and I believe now that it's simpler than I thought it was. Here's my explanation, beginning with a clarification of the premises to be found in the passage:

• Descartes' critics say that the extensions of mathematical objects (i.e., extended, physical things) may be nothing more than fictions of my own mind
• So maybe all there is is my thoughts
• This means that my alleged physics is nothing more than pure math
• It follows that everything we can conceive is fictitious
  • Why is everything? We conclude that there is a God, and that he's not an extended thing. Reason tells us that he is not a deceiver. We then go on (qua meditators) to reason our way toward the existence of the external world. The light of reason brings us to this conclusion.
      If the light of reason is wrong, then reason only leads us to falsity, even when we are being as careful as we can possibly be! But reason lead not only to God; it also lead to the cogito. So if reason can be flawed even when we are using it correctly (i.e., by using Descartes' method, described in the 4th Rule, in Rules), then it seems we ought to doubt everything and reach no certainty whatsoever.
  • So it follows from this that everything we can conceive of may be false. Now assume that it is. Then if we are to assent to anything at all, and keep in mind that reason is thus flawed, we will be assenting only to those things we don't recognize as true (because we can doubt them)! This is so absurd that it amounts to saying that we ought only assent to only those things we don't conceive. Now this is downright impossible. So we won't be using our reason at all in our judgments (because reason tells us we can doubt everything). If we don't use our reason in judgment, then we might as well be animals without reason. For other animals have no soul, and thus no reason.
  • So for all intents and purposes, we are nothing over and above monkeys or parrots.
  • So this seems to be a reductio ad absurdum of the claim that reason may be unreliable.