 The distinction between (or relation or composition of) quiddity and existence is a fundamental issue of Avicenna’s ontology. This holds too for the modal concepts that connote it in as much that quiddity could be said to refer to possibility, whereas existence would be necessity. Although problematic (especially as regards mental existence), it allows Avicenna to give an account of the so-called “ontological difference” between the uncaused Principle and the caused world: the First Principle is absolutely necessary and simply coincides with, or more exactly, is Its own existence: “that is” expresses a primary attribute of the Principle (all others are relations, either positive or negative: Ilāhiyyāt, VIII, 7, 367, 13–15; cf. 4, 343, 13–344, 5; 5, 354, 12–14), whereas in everything else there is a duality. In every thing the distinction between what the thing is and the fact that it is is inevitable. Existence can consequently be said to be external to essence, so that an existing thing, whose essence or quiddity is possible, can be said to be composed of essence and existence. Conversely, in that which is in itself necessary there is no need for such a composition (there is no essence: no being something, but only being). Necessity is an affirmation or more precisely a confirmation of existence (taʾakkud al-wuǧūd: Ilāhiyyāt, I, 5, 36, 4–5). The idea of a distinction/composition thus has a precise theological function: not only does it distinguish everything from the Necessary Principle (everything except the Principle is twofold: Ilāhiyyāt, I, 7, 47, 18–19); it also reveals the divine creative act: everything except the Principle receives (or obtains) existence (Ilāhiyyāt, VIII, 3, 342, 10; Taʿlīqāt, 175, 10–176, 7). The distinction also explains the derivation of celestial souls and bodies from supernal intelligences: in this respect emanation is nothing but the exposition of the dialectic between necessary existence and possible essence, which together constitute the intelligences’ being. This same distinction between a possible essence and its necessary existence can, finally, explain any causal relationship: a cause is ultimately nothing but that which allows the transition from the possible to the necessary, or that which makes possibility incline towards necessity. Certainly, a cause may, in its turn, be possible and so refer to a further cause, but every causal chain leads back to the First Cause, which is in Itself necessary and one (Ilāhiyyāt, VIII, 1–3).