In his An Essay Concerning Human Understanding, Locke’s primary aim is to provide an empiricist theory of ideas that can support interesting results about the nature of language and knowledge. Within this theory, Locke distinguishes between simple ideas and complex ideas (E II.ii.1: 119). Roughly, an idea is complex if it has other ideas as parts; otherwise, it is simple. For Locke, as is well known, all simple ideas derive from sensation (perception through sight, taste, smell, hearing, or touch) or reflection (a form of introspection directed at mental acts) (E II.i.2–4: 104–106). Aetiology also plays a role in Locke’s classification of complex ideas: ideas of modes, ideas of substances, and ideas of relations. All complex ideas are formed by a voluntary act of combination or composition. Ideas of modes, such as numbers, beauty, and theft (E II.xii.5: 165) are formed without considering whether the combinations conform to real patterns existing in the world (E II.xi.6: 158, E II.xxii.1: 288, E II.xxxi.3: 376). Ideas of substances (such as human beings, sheep, and armies—E II.xii.6: 165), by contrast, are formed with a desire ‘to copy Things, as they really do exist’ (E II.xxxi.3: 377). Ideas of relations are like ideas of modes (E II.xxxi.14: 383–84), except that their aetiology includes, in addition to the mental act of composition, the distinct mental act of comparison on the basis of some respect or dimension (E II.xi.4: 157, E II.xxv.1: 319).