Seminal philosophical work published by French philosopher Réné Descartes (in Latin) in 1641. The "first philosophy" in the title of this work pertains to the fundamental foundations of philosophical (especially metaphysical) inquiry - until this point the Western thinkers had been labouring on the stately but showing-its-age platform of Aristotlean thought, which had granted served as a good alternatitve to Platonism for hundreds of years but was holding back modern science (condemning Copernicus for heresy, for instance) at the encroaching Renaissance pawing and growling at the gate. As conventionally matters of metaphysics and epistemology had been considered foundational for philosophical thought, Descartes decided to shed over a thousand years of baggage and give those fields a hard boot.

This wrenched the direction of philosophical inquiry from "Can I know how many angels can dance on the head of a pin?" to "Is there an 'I' to know anything?" (He resolves that latter question, by the by, in his famous assertion cogito, ergo sum - claiming that if there is the wondering thought "Do I exist?" that there must be a thinker to have the thought - however, that famous phrase does not appear in this work, "Cogito, sum" being about as close as he gets.)

Though the Meditations contain much which seems to be silly to a contemporary audience (such as the (surprisingly gnostic) hypothetical omnipotent demonic deceiver who exists only to make Descartes think things are a different way from the way they actually are) and some of his arguments and "proof"s spaghetti back on themselves, the work ultimately succeeded in its goal - not necessarily of establishing its answers as the foundations for a new philosophy, but in opening the doors to new forms of inquiry in opposition to the Classical mode of thought which had not yet been supplanted. Opening the door a crack for himself encouraged subsequent foot traffic and soon all sorts of people, notably Spinoza and Kant, were thinking and writing all sorts of clever, devilishly interesting and heretical things which couldn't be conceived of under the clunkety Aristotlean patch job.

Prefatory Note To The Meditations
Dedication to Meditations on First Philosophy
Synopsis of the Six Following Meditations
Meditation I: Of the Things which may be brought within the Sphere of the Doubtful
Meditation II: Of the Nature of the Human Mind; and that it is more easily Known than the Body
Meditation III: Of God: that he Exists
Meditation IV: Of the True and the False
Meditation V: Of the Essence of Material Things, and, again, of God, that he Exists
Meditation VI: Of the Existence of Material Things, and of the real Distinction between the Soul and Body of Man

The version I have posted here contains (from newest to oldest and from readable-by-the-most-people-here to readable-by-the-fewest) the full text of the 1901 English translation of the work by John Veitch (with amendments / restorations from previous translations indicated in visible hard brackets), the 1647 French translation by le Duc de Luynes and Descartes' original Latin version for full cross-referencing. As page numbers are not terribly useful in an online text, they have been removed and each paragraph clearly marked as P1. etc. for comparative and referential purposes.

If you have anything to add to or challenge the topic or what I've asserted above, please do; I'm more than a bit loopy after spending all day formatting.