 In all of this Fârâbî is broadly following the Aristotelian idea that, for any composite object X, we can ask “what is X?” and answer the question either by giving a definition of X spelling X out into many in-some-sense-constituents of X, or by giving some one such constituent of X, which is a partial answer to “what is  X?”.[28]  Fârâbî is also following Aristotle in insisting that all of the things which should be mentioned in the definition of X are causes of X’s being (“the thing by which [X’s] constitution [is]”), and, furthermore, essential causes, in the sense that it is not simply that some instance of X happens for this reason, but that a thing wouldn’t be X if it weren’t caused in this way: according to Aristotle in Metaphysics VIII,4, a scientific definition of X should mention all and only such causes of X. He is also following Aristotle in saying that this process must come to a stop with simples, where there will be no further definition and no distinction between the thing and any further answer to “what is it?”. He also follows Aristotle further down, in Book of Letters III,244, in saying that in asking for the cause of X’s being, we cannot look for a middle term between the subject and predicate of “X is [X mawjûd]”, or between the subject and predicate of “X is X”. Rather, we must unpack this judgment so that its subject and predicate are different, so that we can seek a middle term connecting them (“why are there eclipses?” might be unpacked as “why is the moon eclipsed?” or “why is the moon darkened at opposition?”): if X is simple, so that “X is” cannot be unpacked into a judgment with distinct subject and predicate, then there is no further investigation into why X is, and therefore also not into what X  is.[29]  But Fârâbî is putting all of this in the service of an argument which has no direct parallel in Aristotle, that, contrary to the linguistic appearance of the paronymous term mawjûd (being or existent), a thing does not exist through an accident of existence [wujûd] as it is white through an accident of whiteness (so I,84, p.113,9–14, cited above). If we say that X exists [mawjûd] in the sense that X is true, then the existence [wujûd] of X is not an attribute of the real thing X outside the mind, but only an attribute of the mental concept of X, namely, that that concept is instantiated outside the mind. If we say that X exists [mawjûd] in the sense that X is circumscribed by a real quiddity, then the existence [wujûd] of X is that quiddity: either it is the whole essence of X as spelled out by its definition, or it is some part of the essence of X, or if X is simple it is just X itself, but in no case is it an accident of X. (Fârâbî also says similar things about two-places uses of mawjûd as a tenseless copula “X is Y”: particularly in I,101 he argues that “X is Y” has a sense which does not assert, e.g., that the quiddity of X is to be Y, or that either X or Y have real quiddities outside the mind. What it asserts is something like two-place being-as-truth, but Fârâbî says that such a sentence need not assert that something is outside the mind as it is in the mind, since in some “X is Y” is true although neither X nor Y exists outside the mind.)