René Descartes
Doubt everything to find what survives doubt.
Discourse on the Method essential
Published anonymously in Leiden, in French, to be read by everyone, not only scholars. A preface to three scientific essays.
Meditations on First Philosophy essential
In Latin, published with six (later seven) sets of objections and his replies. The French translation by the Duke of Luynes (1647) was reviewed by Descartes himself.
Principles of Philosophy essential
A textbook in numbered articles. Picot's French translation (1647) was reviewed by Descartes, who added a famous preface (the "tree of philosophy").
The Passions of the Soul essential
Born of his correspondence with Elisabeth of Bohemia: how soul and body are united, and how to master one's emotions.
The World / Treatise on Man
Written around 1630-1633 but withheld after Galileo's condemnation (it defends the Earth's motion). Published 14 years after his death.
Rules for the Direction of the Mind
Written around 1628, unfinished. Found among his papers, published in Dutch (1684) then Latin (1701).