Descartes's major work, the Meditations on First Philosophy (1641) are considered by many the first text of modern philosophy. Divided in six agile chapters, they provide a new foundation for philosophical and scientific thinking, addressing issues in epistemology (skepticism in particular), metaphysics, theology, and ethics.
Some Context
The first edition of the Meditations appeared in Latin in 1641 and then in French translation in 1647. At the time, Descartes was forty-five and at the peak of his scientific and philosophical endeavors. The memories of Galileo's condemnation for heliocentrism (1633) were still fresh and it was probably out of fear of coming under the attack of the most notable and powerful theologians that Descartes dedicated his volume to "the most wise and illustrious the Dean and Doctors of the Sacred Faculty of Theology in Paris."
The text recounts of six meditations over six successive days, thus divided into six chapters. This is a clever rhetorical device, whose effect is to focus the reader's attention on Descartes's philosophy without having to rehearse the multiple views he was opposing. It is only in an appendix to the main text - the Objections and Replies - that the reader is exposed to other philosophies; these addenda are a crucial part of the whole text and they were published along with it since its first edition.
Meditation I: Three Skeptical Doubts
The first meditation presents the reader with three skeptical doubts. (i) The first regards the senses. Can you really trust your eyesight when it comes to judge - say - the shape or size of things? Can you trust your smell when it comes to judge what a food tastes like? How about the hearing and the other senses? (ii) The second concerns the very existence of material objects. Are you really awake at this time, reading an article by your About.com guide Andrea Borghini, or are you sound asleep? Don't dreams look as veridical as the experience you are going through presently? How can you rule out that your whole life is but a dream? (iii) Even more radically, what if you were but the creation of an omnipotent evil deceiver, who 'engineered' you so to systematically get every of your reasoning wrong, including algebra, geometry, and logic?
Meditation II: the Cogito and Material Substances
In the second meditation Descartes finds the first step out of the skeptic's trap. I have to exist, since I am thinking. This is an immediate intuition for him, evident "by natural light." From it follows that we know of our own existence independently of our knowledge of our own body since we know that we exist even while supposing that we do not have a body. The consequence (which will be drawn in the sixth meditation) is that our mind and body are distinct substances. In just a few passages Descartes had thus laid out the presuppositions of his mind-body dualism: this is where modern philosophy starts.
Meditation III: Proofs of God's Existence
After proving that I have to exist, even while supposing that nothing else is certain, Descartes makes a very audacious move: from my thinking I can prove nothing less than God's existence. How? Well, go through your ideas. Some certainly were derived from things you experience from the senses: pineapple taste, for example. Some ideas can be fancied by mixing and matching your experiences; for example, unicorns or the creatures in the TV series Visitors. But the idea of God could not have been produced by anything less perfect than God. How else could you have derived it? In this meditation Descartes actually offers two different proofs, the second however being a variant of the first.
Meditation IV: Free Will
The fourth meditation almost breaks the flow of the argumentation. Its goal is to prove that humans are free agent and that God is not responsible of the errors that we run into. Here is why. God gave us a finite intellect and an infinite will. If we use them well, we can avoid running into any error. How is this possible? Simple: avoid judging of that which you do not clearly and distinctly understand. Errors, including evil which proceeds from human volition, is originated when our will brings us to judge of those things which we do not understand, thus leading us to do things whose consequences we cannot entertain. Finally, God is not to blame for having provided us with a finite intellect, because there is no imperfection per se in what is finite.
Meditation V: Dispelling the Doubts
At the outset of the fifth meditation Descartes dispels the third doubt: God's perfection is incompatible with the hypothesis of God deceiving us, since deception is a shortcut for imperfect beings. From this it follows that I can trust my mathematical reasoning, if I have sufficient evidence for my results. And I can trust that I am not dreaming - washing away the second doubt - if I have sufficient evidence to think that I am not.
Meditation VI: the Problem of Theodicy
Finally, in meditation six Descartes proves that I can trust my senses, under appropriate circumstances. However, there is a problem. If God is infinitely good and omnipotent, how come there are some innocent people dying because of the errors deriving from their senses? Think for example to those afflicted by dropsy. If God is omnipotent, couldn't God have bestowed upon us senses that were not prone to such an illness? If God is infinitely good, how come God tolerates an illness that kills innocents? The justification of God's omnipotence and infinite benevolence in light of the existence of evil goes under the name of theodicy. Theodicy is a key chapter of modern theology, and Descartes can be considered as its modern founder.
Objections and Replies
The Meditations were published along with seven sets of objections and replies. Two of those (the second and the sixth) collected questions from several authors. The other ones came from important figures of the philosophical and scientific European panorama: the Dutch theologian Johannes Caterus; friar Marin Mersenne; the great British philosopher and social contract theorist Thomas Hobbes; the leading mathematician Antoine Arnauld; the French empiricist Pierre Gassendi; the Jesuit Pierre Bourdin.
Online Sources
An online edition of the Meditations:
http://www.earlymoderntexts.com/f_descarte.html
The entry on Descartes at the Stanford Encyclopedia of Philosophy:
http://plato.stanford.edu/entries/descartes/
