 The most basic questions about the Book of Letters are what, if anything, the treatise is about overall, and how the different parts of the treatise contribute to knowledge of that aim. Mahdi 1969a divides the text into three “Parts” [abwâb]; in the manuscripts that have been studied so far, the three Parts are probably not transmitted in the right order, some parts of the text and perhaps in particular an introduction are missing, and it is not obvious what the subject-matters of the three Parts have to do with each other. The title Kitâb al-ḥurûf = Book of Letters is the same title by which Fârâbî in On the Aims cites Aristotle’s Metaphysics (which presumably gets called Book of Letters because Arabic authors cite its individual books by the names of letters of the alphabet, just as we do now), so the title suggests that Fârâbî intends this treatise to correspond to the Metaphysics; and there are heavy overlaps of content between the Book of Letters and Aristotle’s Metaphysics, especially but not exclusively Book V, which make it very unlikely that the correspondence of titles is a coincidence. But the title Kitâb al-Ḥurûf can also mean something else, since “ḥurûf” (sg. ḥarf), besides meaning “letters”, can also mean grammatical “particles”, i.e., any words that can neither be declined like a noun nor conjugated like a verb, so pronouns and articles and prepositions and adverbs and  conjunctions.[16]  Parts I and III of the Book of Letters are organized “lexically” (like Fârâbî’s On Intelligence), around the many meanings of each of a series of terms, and the majority of the terms here are grammatically particles rather than nouns or verbs: once again, it seems very unlikely that this is a coincidence. But how can the same book be Kitâb al-Ḥurûf both in the sense of filling the role of Aristotle’s Metaphysics, and in the sense of being about grammatical particles? And how would a study of the meanings of particles (and of some selected nouns) contribute to metaphysics as Fârâbî has described it in On the Aims? To see how this could work, we have to see something about the projects of the three Parts of the Book of Letters and how they fit together; and we have to start with Part II, which Fârâbî probably intended to come first, and which can be taken as programmatic for the whole treatise. After discussing the project of the treatise, we will focus on its most remarkable claims about the different senses of being [mawjûd] and the corresponding senses of existence [wujûd], and about how they should be investigated, and the implications for metaphysics. Finally, we will look more briefly at the corresponding claims about one and unity in On the One and Unity.