Meditations on First Philosophy (Latin: Meditationes de prima philosophia, in qua Dei existentia et animae immortalitas demonstrantur) by Réne Descartes was first published in Latin in 1641, appearing in a French translation in 1647.
The six meditations which make up the book describe a series of mental exercises, undertaken over consecutive days. The first meditation introduces Descartes' method of universal doubt. The second introduces the famous argument often summarised as 'I think therefore I am' (Latin: cogito ergo sum), and cannot doubt my own existence.
In the later meditations, Descartes arrives at conventional opinions about God and the world, while more subtly introducing the foundations of his own system of physics. It is however, the first two meditations which have more often been seen as a foundational influence on modern philosophy, although the 'Cartesian dualism' which they introduced between mind and matter has been a target of persistent criticism.
Alongside the Meditations, Descartes published seven sets of objections by distinguished scholars along with his replies. These were 1. Johannes Caterus 2. Marin Mersenne 3. Thomas Hobbes 4. Antoine Arnauld 5. Pierre Gassendi 6. Further objections collected by Mersenne. 7. Pierre Bourdin.
Free online texts
The Classical Library: The Meditations, translated by John Veitch. HTML format.
Gallica: Méditations métaphysiques. French text (1690). Image file format.
Gutenberg: Meditationes de prima philosophia - Latin text. Multiple formats.
Early Modern Texts: Meditations on First Philosophy - adapted and translated into modern English, by Jonathan Bennett. PDF format.
Internet Archive: Rules for the Direction of the Mind, Discourse on the Method, Meditations on First Philosophy, Objections against the Meditations and Replies, The Geometry, by René Descartes. The Ethics, by Benedict De Spinoza. Great Books of the Western World, no 31 (1925). Multiple formats.
Latin Library: Meditationes - Latin text. HTML format.
Liberty Fund: The Method, Meditations and Philosophy of Descartes, translated by John Veitch. Multiple formats.
Marxists.org: Meditations on First Philosophy, translated by John Cottingham. HTML format.
Philosophy-Index: Meditations on First Philosophy, translated by John Veitch. HTML format.
University of Leeds/Internet Archive: Hobbes' Objections to Descartes' Meditations. HTML format.
Wright State University: Descartes' Meditations. English, French and Latin texts. HTML format.