On Why He Couldn’t Assert Immortality Of The Soul Immediately:
But because some people will perhaps expect to see proofs for the immortality of the soul in this [second] Meditation, I think they should put on notice here that I have attempted to write only what I have carefully demonstrated…….
(Meditations On First Philosophy, Synopsis, page 54)
[Note: Descartes has to spend a lot of time placating the crazies]

On The Relation Between Philosophy And Theology:
I have always thought that two issues – namely, God and the soul – are chief among those that ought to be demonstrated with the aid of philosophy rather than theology.
(Meditations On First Philosophy, Dedication, page 47)

On Atheism:
Granted, it is altogether true that we must believe in God’s existence because it is taught in the Holy scriptures, and, conversely, that we must believe the Holy Scriptures because they have come from God. This is because, of course, since faith is a gift from God, the very same one who gives the grace that is necessary for believing the rest can also give the grace to believe that he exists. Nonetheless, this reasoning cannot be proposed to unbelievers because they would judge it to be circular.
(Meditations On First Philosophy, Dedication, page 47)

Moreover, I know that there are many irreligious people who refuse to believe that God exists and that the human mind is distinct from the body – for no other reason than their claim that up until now no one has been able to demonstrate these two things.
(Meditations On First Philosophy, Dedication, page 48)

I have seen two rather lengthy treatises [that respond to Discourse On Method], but these works, utilizing as they do arguments drawn from atheist commonplaces, focused their attack not so much on my arguments regarding these issues, as on my conclusions. Moreover, arguments of this type exercise no influence over those who understand my arguments, and the judgments of many people are so preposterous and feeble that they are more likely to be persuaded by the first opinions to come along, however false and contrary to reason they may be, than by a true and firm refutation of them which they hear subsequently. Accordingly, I have no desire to respond here to these objections, lest I first have to state what they are. I will only say in general that all the objections typically bandied about by the atheists to assail the existence of God always depend either on ascribing human emotions to God, or on arrogantly claiming for our minds such power and wisdom that we attempt to determine and grasp fully what God can and ought to do. Hence these objections will cause us no difficulty, provided we but remember that our minds are to be regarded as finite, while God is to be regarded as incomprehensible and infinite.
(Meditations On First Philosophy, Preface, page 52)

On The Certainty Of His Meditations And Their Potential For Misinterpretations:
Although the arguments I use here do, in my opinion, equal or even surpass those of geometry in certitude and obviousness, nevertheless I am fearful that many people will not be capable of adequately perceiving them, both because they are a bit lengthy, with some of them depending on still others, and also because… they demand a mind quite free from prejudices and that can easily withdraw itself from association of the senses. Certainly there are not to be found in the world more people with an aptitude for metaphysical studies than those with an aptitude for geometry.
(Meditations On First Philosophy, Dedication, page 49)

On Teleology:
…the entire class of causes which people customarily derive from a thing’s “end,” I judge to be utterly useless in physics. It is not without rashness that I think myself capable of inquiring into the ends of God.
(Meditations On First Philosophy, Meditation 4, page 82)

On Truth And Clarity And Certainty:
I am certain that I am a thinking thing. But do I not therefore also know what is required for me to be certain of anything? Surely in this first instance of knowledge, there is nothing but a certain clear and distinct perception of what I affirm. Yet this would hardly be enough to render me certain of the truth of a thing, if it could ever happen that something that I perceived so clearly and distinctly were false. And thus I now seem able to posit as a general rule that everything I very clearly and distinctly perceive is true.
(Meditations On First Philosophy, Meditation 3, page 70)

[The idea of God discloses to me something real… it is] an idea that is utterly clear and distinct… for whatever I clearly and distinctly perceive to be real and true and to involve some perfection is wholly contained in that idea.
(Meditations On First Philosophy, Meditation 3, page 77)

But once I perceived that there is a God, and also understood at the same time that everything else depends on him, and that he is not a deceiver, I then concluded that everything that I clearly and distinctly perceive to be true. Hence even if I no longer attend to the reasoning leading me to judge this to be true, so long as I merely recall that I did clearly and distinctly observe it, no counter-argument can be brought forward that might force me to doubt it. On the contrary, I have a true and certain knowledge of it. … For what objections can now be raised against me? That I have been made such that I am often mistaken? But I now know that I cannot be mistaken in matters I plainly understand. That I have taken many things to be true and certain which subsequently I recognized to be false? But none of these were things I clearly and distinctly perceived. But I was ignorant of this rule for determining the truth, and I believed these things perhaps for other reasons which I later discovered were less firm.
(Meditations On First Philosophy, Meditation 5, page 92)
[Note: Notice how God’s existence ties in (precedes?) this understanding of truth as clarity.]

On God’s Existence:
In fact the idea I clearly have of the human mind – insofar as it is a thinking thing, not extended in length, breadth or depth, and having nothing else from the body – is far more distinct than the idea of any corporeal thing. And when I take note of the fact that I doubt, or that I am a thing that is incomplete and dependent, there comes to mind a clear and distinct idea of a being that is independent and complete, that is, an idea of God. And from the mere fact that such an idea is in me, or that I who have this idea exist, I draw the obvious conclusion that God also exists, and that my existence depends entirely upon him at each and every moment. This conclusion is so obvious that I am confident that the human mind can know nothing more evident or more certain.
(Meditations On First Philosophy, Meditation 4, page 81)

However, as far as God is concerned, if I were not overwhelmed by prejudices and if the images of sensible things were not besieging my thought from all directions, I would certainly acknowledge nothing sooner or more easily than [God].
(Meditations On First Philosophy, Meditation 5, page 91).

For just as the objective mode of being belongs to ideas by their very nature, so the normal mode of being belongs to the causes of ideas, at least to the first and preeminent ones, by their very nature. And although one idea can perhaps issue from another, nevertheless no infinite regress is permitted here; eventually some first idea must be reached whose cause is a sort of archetype that contains formally all the reality that is in the idea merely objectively.
(Meditations On First Philosophy, Meditation 3, page 74)

On Why One Can Invent Corporeal Ideas, But Not Divine Ideas:
As to the ideas of corporeal things, there is nothing in them that is so great that it seems incapable of having originated from me… There is another kind of falsity (called “material” falsity) which is found in ideas whenever they represent a non-thing as if it were a thing… Assuredly I need not assign to these ideas an author distinct from myself. For if they were false, that is, if they were to represent non-things, I know by the light of nature that they proceed from nothing; that is, they are in me for no other reason that something is lacking in my nature. If, on the other hand, these ideas are true, then because they exhibit so little reality to me that I cannot distinguish it from a non-thing, I see no reason why they cannot get their being from me.
(Meditations On First Philosophy, Meditation 3, page 75

On “The Unicorn Defense”:
From the fact that I think of God as existing, it does not seem to follow that God exists, for my thought imposes no necessity on things. And just as one may image a winged horse, without there being a horse that has wings, in the same way perhaps I can attach existence to God, even though no God exists. But there is a sophism lurking here. From the fact that I am unable to think of a mountain without a valley, it does not follow that a mountain or a valley exists anywhere, but only that, whether they exist or not, a mountain and a valley are inseparable from one another. But from the fact that I cannot think of God except as existing, it follows that existence is inseparable from God, and that for this reason he really exists.
(Meditations On First Philosophy, Meditation 5, page 89)

On The Unreliability Of The Senses:
Many experiences gradually weakened any faith that I had in the senses… [examples include phantom limb pain and flaws in visual recognition]… To these causes for doubt I recently added two quite general ones. The first was that everything I ever thought I sensed while awake I could believe I also sometimes sensed while asleep, and since I do not believe that what I seem to sense in my dreams comes to me from things external to me, I saw no reason why I should hold this belief about those things I seem to be sensing while awake. The second was that, since I was ignorant of the author of my origin (or at least pretended to be ignorant of it), I saw nothing to prevent my having been so constituted by nature that I should be mistaken even about what seemed to me most true.
(Meditations On First Philosophy, Meditation 6, page 95)

Thus what I thought I had seen with my eyes, I actually grasped solely with the faculty of judgment, which is in my mind.
(Meditations On First Philosophy, Meditation 2, page 68)

On Why “Everything That Begins To Exist Has A Cause” Includes The Non-Corporeal:
Now it is indeed evident by the light of nature that there must be at least as much [reality] in the efficient and total cause as there is in the effect of that same cause. For whence, I ask, could an effect get its reality, if not from a cause? And how could the cause give that reality to the effect, unless it also possessed that reality? Hence it follows that something cannot come into being out of nothing, and also that what is more perfect (that is, what contains in itself more reality) cannot come into being from what is less perfect. But this is manifestly true not merely for those effects whose reality is actual or formal, but also for ideas in which only objective reality is considered… As imperfect a mode of being as this is by which a thing exists in the intellect objectively through an idea, nevertheless it is plainly not nothing, hence it cannot get its being from nothing.
(Meditations On First Philosophy, Meditation 3, page 73-74)

On Thought And Forms:
Rather, the very nature of an idea is such that of itself it needs no formal reality other than what it borrows from my thought, of which it is a mode.
(Meditations On First Philosophy, Meditation 3, page 76)

I am a substance.
(Meditations On First Philosophy, Meditation 3, page 76)

On Self-Knowledge Never Being Wrong:
Now as far as ideas are concerned, if they are considered alone and in their own right, without being referred to something else, they cannot, properly speaking, be false. For whether it is a she-goat or a chimera that I am imagining, it is no less true that I imagine the one than the other. Moreover, we need not fear that there is falsity in the will itself or in the affects, for although I can choose evil things or even things that are utterly non-existent, I cannot conclude that it is untrue that I do choose these things. Thus there remain only judgments in which I must take care not to be mistaken.
(Meditations On First Philosophy, Meditation 3, page 71)
[Note: Above passage seems to hint at an is-ought distinction.]