Meditations on First Philosophy (subtitled In which the existence of God and the real distinction of mind and body, are demonstrated), written by René Descartes (1596 - 1650) and first published in 1641, expands upon Descartes' philosophical system, which he first introduced in his Discourse on Method (1637).
The book is made up of 6 meditations, during which Descartes discards all belief that is not absolutely certain, then tries to establish what can be known for sure.
Thus, the first meditation contains arguments for doubting and philosophical skepticism; the most famous of which is that a malign demon might be systematically deceiving you at all times (compare with the modern equivalent, the brain-in-a-vat theory).
The second meditation contains Descartes' argument for the certainty of one's own existence, even if all else is in doubt:
The rest of the book contains arguments that modern philosophers have found less convincing, such as ontological arguments for the existence of God, and the supposed proof of the dualism of mind and matter.