 Fârâbî further develops the discussion of the relation between a being and its existence, in different senses of “existence”, in Book of Letters III. Here he is officially continuing the discussion of particles by explaining the meanings of various interrogative particles in different “syllogistic arts”: the main interest is to explain the distinctive meanings these particles have in demonstrative science, and to warn against confusing them with the meanings they have in other disciplines. This resembles the discussion in Book of Letters I, but the particles (and “logical” particles which are grammatically nouns in Arabic) discussed in Book of Letters I correspond roughly to those discussed in Aristotle’s Metaphysics V; Book of Letters III, by contrast, picks up issues from Posterior Analytics II. There Aristotle discusses the different kinds of scientific “question” or “investigation” [zêtêsis], and he specifies each one by an interrogative particle (“whether”, “what”, “why”). But Aristotle is interested mainly in the methods the scientist uses to answer these questions; nobody before Fârâbî had thought there was a distinctive scientific meaning of the particles themselves. But Aristotle does talk about how the scientist should investigate whether something is, what something is, and why something is, and Fârâbî takes this discussion as giving not just a methodology but a scientific account of existence and quiddity, i.e., of the scientific meanings of the particles “whether” and “what”.