Meditations on First Philosophy (subtitled In which the existence of God and the immortality of the soul are demonstrated) is a philosophical treatise by René Descartes first published in 1641 (in Latin). The French translation (by the Duke of Luynes with Descartes' supervision) was published in 1647 as Méditations Metaphysiques. The original Latin title is Meditationes de prima philosophia, in qua Dei existentia et animæ immortalitas demonstratur. The title may contain a misreading by the printer, mistaking animae immortalitas for animae immaterialitas, as suspected already by A. Baillet. 
The book is made up of six meditations, in which Descartes first discards all belief in things that are not absolutely certain, and then tries to establish what can be known for sure. He wrote the meditations as if he had meditated for six days: each meditation refers to the last one as "yesterday" (In fact, Descartes began work on the Meditations in 1639.) One of the most influential philosophical texts ever written, it is widely read to this day.
The Meditations consist of the presentation of Descartes' metaphysical system in its most detailed level and in the expanding of Descartes' philosophical system, which he first introduced in the fourth part of his Discourse on Method (1637). Descartes' metaphysical thought is also found in the Principles of Philosophy (1644), which the author intended to be a philosophy guidebook.