 Aristotle discusses there four types of scientific question or investigation, “whether X is”, “what X is”, “whether X is Y”, and “why X is Y”. His main claim is that the investigation what X is stands to the investigation whether X is, as the investigation why X is Y stands to the investigation whether X is Y. So in order to determine what X is, we must first investigate whether X is, and then, if the answer is yes, investigate why X is. An answer to why X is can be converted into a scientific answer to what X is; where to discover why X is, we must reformulate the judgment “X is” as a two-place judgment with distinct subject and predicate terms, and then look for a middle term connecting them. In the standard example, if X is (lunar) eclipse, before we can scientifically investigate what eclipses are, we must first investigate whether there are eclipses; if there are, and if the nominal definition of (lunar) eclipse is “darkening of the moon at opposition”, then we investigate why the moon is sometimes darkened at opposition; and if we conclude that this is due to the interposition of the earth between the moon and the sun, we can convert this into a scientific definition of eclipse as “darkening of the moon at opposition due to the interposition of the earth between the sun and the moon”. Fârâbî takes all this up in Book of Letters III,226 and following, under the headings of the scientific meanings of the particles “whether”, “what”, and “why”: or rather, when he says “whether X exists” (or “does X exist?”, hal X mawjûdun), both the word “whether” and the word “exists” are logically, if not grammatically, particles. Fârâbî follows Aristotle in saying that there is an ordered series of questions or investigations: first we must ask whether X exists, and only then, if the answer is yes, can we ask what X is, by asking why X exists. But Fârâbî’s story is more complicated than Aristotle’s, because he distinguishes different senses both of “whether X exists” and of “what X is”. Fundamentally, Fârâbî thinks that there is a series of four questions: first “what X is” seeking the nominal definition of X, which spells out the concept of X in the soul; then “whether X is” in the sense of being-as-the-true, asking whether X exists outside the soul as it does in the soul; then “whether X is” in the sense of being as having a quiddity outside the soul, asking rather “does it have some thing by which its constitution [qiwâm] [is] and which is present in it” (III,229 p.214,6–7, cited above); then “what X is” seeking the quiddity of X outside the soul, namely this thing present in X which is the cause of X’s constitution. The Posterior Analytics has nothing corresponding to the distinction between the two “whether X is” questions, but Posterior Analytics II,2 does say that when we ask/investigate whether X is Y, or whether X is, we are asking “whether there is a middle term for it”, and that when we then ask/investigate why X is Y, or what X is, we are asking “what that middle term is” (89b37–90a1). And Fârâbî argues that this requires a distinctive meaning of “is” in the question “whether X is”, since X could exist in the sense that it is outside the soul as it is in the soul, without there being a middle term between some subject and some predicate which would cause X’s existence. There are at least two ways this could happen. First, if X is a privation or even “privation” in general (see III,240 p.218,12–15, quoted above), or presumably also if X is an ens per accidens like white horse, there simply is nothing outside the mind by which X is constituted. Second, if X has/is a simple quiddity, then “X is” cannot be expanded into a sentence “Y is Z” where there might be a middle term between the subject and the predicate. So once we have established that X exists outside the soul, we must still investigate whether X is constituted by some cause outside the soul, and then what that cause is.