 As we saw in  Section 3.4,  on Fârâbî’s analysis in the Book of Letters there are two main senses of being [mawjûd], with corresponding senses of existence [wujûd]. If “being” [mawjûd] means “being outside the mind as it is inside the mind”, then the subject of which is predicated is a thought in the mind (a concept or judgment), and its existence [wujûd] is its relation to something extramental, and this existence holds univocally of all true thoughts. Clearly this cannot be what Fârâbî means in the Perfect City—it would be pointless to ask, in this sense, whether something else has the same existence [wujûd] that the First has. On the other hand, if “mawjûd” means “being circumscribed by a quiddity outside the mind”, then the subject of which it is predicated is a thing really existing outside the mind, and, according to Book of Letters I,89, its corresponding wujûd is either its quiddity or part of its quiddity. Interpreting wujûd as quiddity does not make sense of the Perfect City’s question whether anything else has the same wujûd as the First: it would be trivially true that no two distinct things could have the same quiddity, or, if we say that the quiddity of a thing is just its form without its matter, it would still be trivially true that no two distinct immaterial things could have the same quiddity. But “part of a quiddity” works, particularly if we reflect on what kind of part of the quiddity of X could be called the wujûd of X, and why. As we saw in  Section 3.4,  Fârâbî in the Book of Letters, following Aristotle’s Metaphysics VII,17, analyzes X’s existence [wujûd = ousia in Aristotle’s Greek] as the cause of the fact that X exists, and, in order to search for this cause as a middle term, tries to reanalyze the existential judgment “X exists” as a predicative judgment, something like “Y is X” where Y is the per se subject of X. If X is entirely simple, then “X exists” cannot be reanalyzed in this way, but in this case there is no cause of the fact that X exists, and X’s existence [wujûd] is identical to X without remainder. In cases where X can be analyzed into a subject and a predicate, one meaning of “X exists” is that there is a middle term explaining why the predicate belongs to the subject: and this middle term (not the linguistic expression but the reality it signifies) is one of the things that can be called “the existence [wujûd] of X by which it is constituted” or “the cause of existence to X” or “a part by which its existence [is constituted]”. In this sense the interposition of the earth between the sun and the moon is a partial existence [wujûd] of eclipse, or a part of the quiddity of eclipse which is a cause of existence to eclipse, or a cause of the constitution of  eclipse.[52]  Since the First is entirely simple, its quiddity cannot be spelled out into a definition, and so the First’s existence is identical to the First without remainder, and there is no way that two simple things can share the same existence. But the First is a “cause of existence” to a Second, or is “part of that by which the existence of the Second is constituted”, and similar expressions, if the First enters as a cause into the real causal definition of the Second, which it does, since (as we will see) the Second is constituted by its act of cognizing the First. This makes sense of the different things Fârâbî says in the Perfect City about X being the cause of existence to Y, or X being a part of the constitution [qiwâm] of Y, of X and Y sharing the same existence. It also helps to show what the extra content is to saying “X is the cause of existence to Y” rather than just “X is the cause of Y”, and helps to explain Fârâbî’s ground for his assumption that there is a First, i.e., that there is not an infinite regress of causes of existence: if X is the cause of Y, X counts as the cause of existence to Y only if X would be mentioned in a causal definition of Y, i.e., if it is essential to Y to be caused by X, so that a Y not caused by X would not really be a Y. If there were an infinite regress of causes of existence to X, it would follow that the causal definition of X would be infinitely long, which Fârâbî presumably regards as absurd, as does Aristotle in Metaphysics II,2. There may well be an infinite regress of causes of coming-to-be, e.g., the infinite chain of my ancestors, but because these are not causes of existence, which would enter into the quiddity of the effect and so would be mentioned in its scientific definition, they do not lead to the absurdity of an infinitely long definition.