 Fârâbî takes this result as indicating that dialectic and demonstrative science give different meanings, not only to the “what is X?” question and thus to quiddity (dialectic seeking a nominal definition, science a causal definition), but also to the “whether X is” question and thus to existence. So he says that, as in dialectic the two-place investigation “whether X is Y” is only concerned with the copula (not with whether it is essential predication), so in dialectic the one-place investigation “whether X is” is asking only about being as the true, i.e., about whether the concept of X is instantiated (III,246–7). He connects this with an inherited problem in Aristotle’s dialectic: in the Topics Aristotle classifies dialectical problems as asking whether Y is an accident of X, whether Y is the genus of X, whether Y is proper to X, or whether Y is the definition of X—so how should we treat the question whether X exists? As Fârâbî says, Alexander of Aphrodisias in his commentary on the Topics classifies questions of existence as questions of accident, treating the existence of X as an accident of X. And Fârâbî says that he is right to do so, since dialectic is asking whether the concept of X in the mind is instantiated outside the mind, and this belongs as an accident to the concept of X. However, if we mean whether the real thing X outside the mind is constituted by some quiddity, or whether X falls under one of the categories, then we would be asking whether something belongs to X as its genus or definition. Fârâbî also says that different particular sciences give different meanings to these questions. Notably, in physics the questions “whether X is” and “what X is” are asking for any kind of cause of X’s existing, whether that cause is internal to X or not (III,237)—it could be an extrinsic efficient or final cause, like the earth blocking sunlight from reaching the moon, which would be incorporated into a physical definition, as in this case of eclipse. Most importantly, in “the divine science”, if we ask whether God is, and if we mean by this whether there is some cause through which God is  constituted,[33]  then while this is a legitimate question the answer can only be no (III,238–9 and III,243). Nonetheless, we can still ask of God “whether he is some circumscribed essence” (III,240, p.218,11), and we can discover that he is, not because we can give a definition analyzing that essence into simpler terms, but because in analyzing other beings and seeking their causal definitions, tracing them back step by step to their causes, we discover that they all lead ultimately to a single first cause of their existence, so that God enters into the fully spelled out quiddities of everything else (III,251). Because we learn in this way that God is simple and dependent on no cause, we can ask “whether his existence is an existence which, in order for him to exist through it, needs nothing other than itself in any way” (III,242 p.220,1–2), neither a substratum nor an efficient cause, and the answer is yes. So too in the divine science, if we ask “whether God is X” for some predicate X (whether God is an intelligence, is knowing, is one, is a maker or cause of something else’s existing, III,242 p.220,2–5), and if we mean by that “whether there is a middle term for it” as Posterior Analytics II,2 suggests, i.e., a cause to God of his being X, such as an accident inhering in God, or a part of God’s essence distinct from the whole essence, then again the answer can only be no. But we can also mean