Hume represented the mind as at least partly constituted by simple perceptions; and, if all complex perceptions consist of simple ones mixed or compounded in various ways, as wholly so constituted. Now there is not much to be said about simple perception in its own right, but apparently more can be said about its relations with other perceptions. So it is not surprising that relations play an important part in Hume's descriptions and explanations of the mental life. But there is difficulty. It has been argued that some at least of the relations of which Hume makes use can only hold between complex things. In this paper, I shall consider whether Hume could have answered this criticism, as it applies to particular part of the Treatise in which he makes perhaps the most extensive use of relations between simple perceptions, the discussions of the passions in Book II. The relations that cause most difficulty are and contrariety. The importance of is emphasized when Hume formulates his principle of association of impressions, which, he says, is analogous to the principle of association of ideas, 'tho with this remarkable difference, that ideas are associated by resemblance, contiguity, and causation; and impressions only by (p. 283).1 As this is his main principle for explaining the occurrence and sequence of passions, the role of is obviously considerable. Contrariety is not so often mentioned in the explanations, but plays an important part in some arguments. Hume calls it a species of resemblance (p. 384), but seems to have no better reason for this than that any two contrary things must resemble each other (p. 15).2 The problems involved in interpreting the two relations are anyway different. These are the relations which I shall mainly discuss, but I shall in the course of the discussion say little about some others, notably relations of degree, certain temporal relations and the relation of cause and effect. But before considering what relations can hold between simple things, we should establish what Hume means by 'simple'.