DESCARTES TO BERKELEY other relations, and it contains an elaborate and highly inXuential discussion of the nature of personal identity Although Locke believes that we can recognize simple ideas within ourselves unaided, and that if we cannot recognize them no words will help us to so, he does in practice identify the ideas that he is talking about by means of the words that express them He admits that ‘our abstract ideas, and general words, have so constant a relation one to another, that it is impossible to speak clearly and distinctly of our Knowledge, which all consists in propositions, without considering, Wrst, the Nature, Use and SigniWcation of Language’ (E, 401) To that topic, then, he devotes his third book The most famous sections of this book are the discussion of abstract ideas and the theory of substance The mind, Locke says, observing likenesses among natural objects, sorts them under abstract general ideas, to which it attaches general names These general ideas have, he tells us, remarkable properties: the general idea of a triangle, for instance ‘must be neither oblique nor rectangle, neither equilateral, equicrural nor scalenon, but all and none of these at once’ Substances in the world possess various qualities and powers which we make use of when we deWne things of diVerent kinds; but the deWnitions we give them not reveal their real essences, but only a ‘nominal essence’ Of substance in general the only idea we have is of ‘something we know not what’ in which properties inhere Epistemological considerations are ubiquitous throughout the Essay, but it is the fourth book that is oYcially devoted to the topic of knowledge Because the real essences of things are unknown to us, we cannot have true science about items in the natural world, but only probable belief We can have genuine knowledge of our own existence and of the existence of God; and provided we keep within the bounds of actual sensation, we can have knowledge of the existence of other things The love of truth should prevent us from entertaining any proposition with greater assurance than the evidence we have for it: ‘Whoever goes beyond this measure of assent, it is plain, receives not truth in the love of it, loves not truth for truth’s sake, but for some other by end’ (E, 697) During his exile, perhaps in 1685 when King Louis XIV revoked the Edict of Nantes which had hitherto given toleration to French Protestants, Locke wrote a Latin letter on toleration (Epistola de Tolerantia) advocating to a European audience, as he had earlier done to an English one, the 52