 Locke makes this distinction in Book II Chapter 8 of the Essay and using Boyle’s terminology calls the two different classes of properties the primary and secondary qualities of an object. This distinction is made by both of the main branches of the mechanical philosophy of the seventeenth and early eighteenth century. Both the Cartesian plenum theorists, who held that the world was full of infinitely divisible matter and that there was no void space, and the atomists such as Gassendi, who held that there were indivisible atoms and void space in which the atoms move, made the distinction between these two classes of properties. Still, the differences between these two branches of the mechanical philosophy affect their account of primary qualities. In the chapter on Solidity (II.4) Locke rejects the Cartesian definition of body as simply extended and argues that bodies are both extended and impenetrable or solid. The inclusion of solidity in Locke’s account of bodies and of primary qualities distinguishes them from the void space in which they move.