Six meditations, one for each day, written in the first person, with the reader instructed to go through the doubts personally rather than watch someone else have them. Published in Latin in 1641 and dedicated, carefully, to the theology faculty of the Sorbonne.

The form is not decoration. The book is built as a sequence — each day dismantles or establishes something the next one needs — and reading it out of order does not work.

What reading it is like

Short. About eighty pages, and the prose is clear, unhurried and unusually personal for philosophy of the period. Descartes writes as a man sitting by a fire, and the first Meditation in particular is among the most readable pages in the subject.

It is also harder than it looks, because the argument is cumulative and the famous parts are load-bearing for parts nobody quotes. The first and second Meditations are widely anthologized on their own and doing that cuts the book where it is most misleading — the doubt and the cogito, without the argument that gets the world back.

The other half of the book is the Objections and Replies, and they are longer than the Meditations themselves. Descartes circulated the manuscript before publication and printed six sets of objections with his answers, from Hobbes, from Arnauld, from Gassendi, and from others collected by Mersenne. A seventh, by the Jesuit Bourdin, was added in 1642.

Nothing else in early modern philosophy has this. You get the argument, the best contemporary attacks on it, and the author's replies, bound together and with the author having chosen the attackers. Almost nobody reads them and they are where the book is genuinely alive — Arnauld's circle objection and Gassendi's materialist pushback are better than most of the secondary literature since.

The six days

One: the doubt. Three waves, each wider. The senses have deceived, so they cannot be trusted absolutely. Dreams are indistinguishable from waking while they last, so the existence of the external world is not secure. And a supremely powerful deceiver could arrange even his arithmetic. The third wave is the innovation: earlier skeptics had the first two.

Two: what survives. However thoroughly he is deceived, he must exist to be deceived. The Latin is ego sum, ego existo — I am, I exist — and the more famous cogito ergo sum is from the Discourse, not from here. He then asks what this thing is, and answers: a thing that thinks. The wax argument follows, arguing that even in a case that looks like pure sense perception, it is the intellect doing the work.

Three: God. The first proof, and it is causal rather than the familiar one. He has an idea of an infinite, perfect being. An idea must have at least as much reality in its cause as it represents. He is finite and cannot have produced it. So something infinite did.

Four: error. Why a non-deceiving God permits mistakes. Because the will extends further than the intellect: we assent to what we do not clearly perceive. Error is a misuse of freedom, not a defect of design.

Five: essences, and the second proof. The ontological argument — existence belongs to the essence of a supremely perfect being as three angles belong to a triangle.

Six: the world. Mind and body are really distinct; the external world exists; the senses are reliable enough for practical life once God's veracity guarantees them. This is where the doubt is finally paid off, and it is the Meditation people skip.

Key passages

Cited by the standard Adam and Tannery volume and page, as all Descartes scholarship is.

First Meditation, AT VII.18–23 — the three grounds of doubt; the evil deceiver.

Second Meditation, AT VII.25 — ego sum, ego existo.

Second Meditation, AT VII.30–33 — the wax.

Third Meditation, AT VII.45–52 — the causal proof from the idea of the infinite.

Fourth Meditation, AT VII.54–62 — will, intellect, and the source of error.

Fifth Meditation, AT VII.65–68 — the ontological argument.

Sixth Meditation, AT VII.78–81 — the real distinction between mind and body.

Fourth Objections (Arnauld) — the circle.

Fifth Objections (Gassendi) — the materialist attack, and the most entertaining exchange in the volume.

What is contested

The circle. The reliability of clear and distinct perception is established by proving God, and the proof proceeds by clear and distinct perception. Arnauld saw it immediately. Descartes answers by distinguishing what is currently perceived from what is merely remembered as having been perceived, and whether that works is still argued.

Whether the cogito is an inference. Therefore suggests a syllogism with a suppressed premise, which would need its own justification. Descartes denies it is an inference and calls it an immediate intuition. See cogito.

Whether the real distinction is proved. Conceiving mind without body establishes that they are conceivable apart. Getting from there to their being separable in reality needs a further principle, and Arnauld pressed exactly this.

Whether the dedication is sincere. The letter to the Sorbonne presents the work as a service to theology. Descartes had watched Galileo's condemnation in 1633 and suppressed his own physics because of it, and how much of the theological framing is conviction and how much is insurance is a live question.