  As for the concept of body, their views are again similar, without being identical. Both confront the problem of a substratum or substance in general, one that Locke notoriously finds  vexing.[26] Newton dispenses with the problem more easily. In De Gravitatione, he eliminates the unintelligible notion of prime matter by associating perceived properties with determined regions of  extension.[27] Both also attack Descartes’ identification of matter with extension, instead defending the void and showing strong sympathy for atomism. As indicated just above, Newton does not confine the quality of extension to body and space, whereas Locke may do so. Further, Newton’s list in Rule 3 of body’s universal qualities  includes not only extension, hardness, impenetrability, and mobility, but also the vis inertiae—the inherent force or power of resisting (Principia, Definition 3), which some commentators take Newton to identify with  mass.[28] Locke’s concept, however, emphasizes extension, mobility, and solidity, without mentioning mass. Still, since Locke explicates solidity in terms of impenetrability, which arises from resistance (E II.iv.1) there may be grounds for holding that his concept does after all include mass, a position defended by Woolhouse  (2005).[29] At the least, Locke’s discussion of action by impulse presupposes mass, as Stein has noted. And yet presupposing the concept may not amount to including it in the concept of body, since as Stein points out, mass for Locke “cannot be construed to correspond to a simple idea, but can only be understood as a power ‘mediately perceivable.’” (Stein 1990, p. 36)