Exactly four hundred years after the birth of René Descartes (1596-1650), the present volume now makes available, for the first time in a bilingual, philosophical edition prepared especially for English-speaking readers, his Regulae ad directionem ingenii / Rules for the Direction of the Natural Intelligence (1619-1628), the Cartesian treatise on method. This unique edition contains an improved version of the original Latin text, a new English translation intended to be as literal as possible and as liberal as necessary, an interpretive essay contextualizing the text historically, philologically, and philosophically, a com-prehensive index of Latin terms, a key glossary of English equivalents, and an extensive bibliography covering all aspects of Descartes' methodology. Stephen Gaukroger has shown, in his authoritative Descartes: An Intellectual Biography (1995), that one cannot understand Descartes without understanding the early Descartes. But one also cannot understand the early Descartes without understanding the Regulae / Rules. Nor can one understand the Regulae / Rules without understanding a philosophical edition thereof. Therein lies the justification for this project. The edition is intended, not only for students and teachers of philosophy as well as of related disciplines such as literary and cultural criticism, but also for anyone interested in seriously reflecting on the nature, expression, and exercise of human intelligence: What is it? How does it manifest itself? How does it function? How can one make the most of what one has of it? Is it equally distributed in all human beings? What is natural about it, and what, not? In the Regulae / Rules Descartes tries to provide, from a distinctively early modern perspective, answers both to these and to many other questions about what he refers to as ingenium.