Rene Descartes

Meditations on First Philosophy: in Which the Existence of God and the Distinction of the Soul from the Body Are Demonstrated Rene Descartes Revised edition

The Meditations has had an incalculable influence on the history of subsequent philosophical thinking. Indeed, according to nearly every history of philosophy you're likely to come across, this work is where modern philosophy begins. It's not that any of Descartes's arguments are startlingly original--many of them have historical precedents--but that Descartes's work was compelling enough to initiate two research programs in philosophy, namely British empiricism and continental rationalism, and to place certain issues (e.g. the mind-body problem, the plausibility of and responses to skepticism, the ontological argument for the existence of God, etc.) on the philosophical agenda for a long time to come. Moreover, Descartes was capable of posing questions of great intrinsic interest in prose accessible to everyone. So the Meditations is a work of value to both newcomers to philosophy and to those with a great deal of philosophical background.