 Fârâbî says that there can be only one such universal science, because sciences are individuated by their subject-domains: if there are two sciences, such that the subject-domain of one includes something that is not within the subject-domain of the other, then at least the second science is not a universal science; so any two universal sciences would have the same subject-domain, and would therefore be the same science. So a universal science, if there is one, would be a science of being as such, and also of unity if (as Fârâbî says, following Aristotle in Metaphysics IV,2) unity is coextensive with being, and also of any attributes (more literally “consequences”) of being or unity, i.e., predicates that can be shown to apply to every existing thing just because it exists or because it is one. (Since Fârâbî cites pairs like “the perfect and the deficient”, he apparently means to include, not just attributes that apply to all beings, but also pairs of attributes that apply disjunctively to all beings, in something like the way that even and odd apply disjunctively to all numbers.) Fârâbî does not argue here that there is a universal science, beyond saying that “it does not belong to any of [the particular sciences] to investigate what is common to all the things that are”. Perhaps his view is that there is at least something to investigate here, namely “what, if anything, is common to all the things that are?”. If that investigation yields no answer, or a negative answer, then there is no universal science; if we discover, for some value of P, that P belongs universally to all the things that are, then that proposition and the arguments for it belong to the universal science, so there is a universal science. Fârâbî is not assuming that being is univocal and so constitutes a single genus. He says in On the Aims that Aristotle in Metaphysics V investigates whether the terms for being and its attributes are univocal or not, and in his Book of Letters (see    Section 3 below) he says that being is not univocal, but is said of substances primarily and of other things derivatively. So if all beings have some attribute P, it will be because substances are P primarily, and other beings are P because substances are; but this is still enough for “all beings are P” to be a theorem of the universal science.