The treatiseDe Rationis Sectione by Apollonius of Perge, which deals with a unique and difficult problem, is a remarkable, complex example of the study of the necessary pre-conditions for the existence of a solution. This volume presents the editio princeps of the text, which has only survived in an Arabic version. It is made accessible in the form of a French translation and a commentary that reveals the mechanisms of Apollonius’ difficult proof, and draws particular attention to its conceptual innovations.