In two separate studies, published some four years apart, Robert McRae has argued the provocative thesis that the idea of extension is not to be numbered among the ideas accounted innate by Descartes, but among the adventitious. He has defended this view despite explicit statements to the contrary by Descartes both in the Correspondence and in the second part of the Principles of Philosophy. Against such evidence McRae has urged the overriding importance of the sixth Meditation, where, he alleges, Descartes asserts “unequivocally… that the idea of extension is produced in us by bodies and is therefore not innate” (147; my emphasis). It is only in the later of the two studies that McRae's reasons for regarding the testimony of the sixth Meditation as authoritative are fully spelt out. Express statements to the contrary “must give away”, he writes, “for the proof of the existence of body by the adventitious idea of it is absolutely crucial to the Meditations” (L 100; my emphasis). In this paper I propose to challenge McRae's interpretation of the extension of the concept “innate idea”, while acknowledging his signal contribution to the clearing up of the intensional meaning, or rather meanings, of the concept of innateness. My thesis is that “extension” is an innate idea, though constraints of time will allow me to do little more than try to show that McRae's stated reasons for rejecting this view are not compelling.