In this paper I shall focus attention on a principle which lies at the heart of Locke's distinction between primary and secondary qualities. It is to be found explicitly or implicitly stated at many places in the Essay , but its clearest expression is at E.II.viii.11, where Locke writes that ' Impulse [is] the only way which we can conceive Bodies operate in'. Let us call it 'the impulse principle'. The first task is to describe what exactly the term impulse means here and to what the principle amounts. Next, I shall consider the kind of role the principle plays in the Essay and whether Locke changed his mind about it in the fourth edition. Then, in the main part of the paper, I shall try to show how the impulse principle helps make possible Locke's distinction between primary and secondary qualities. In the course of my discussion I shall refer to some of Locke's pre- Essay writings: the Epitome, the Abr g , his review of Newton's Principia and Draft C.1 It is a subsidiary aim of the paper to show how these writings - particularly the Abr g which ran to over ninety pages of the Biblioth que universelle and was published in 1688 - can be of help in disentangling the main line of argument in Locke's Essay.