Al-Farabi expresses his theory of causality in the treatise called “the gems of wisdom.” Everything which exists after having not existed, he says, must be brought into being by a cause which itself may be the result of some preceding cause, and so on, until we reach a First Cause, which is and always has been, its eternity being necessary because there is no other cause to precede it, and Aristotle has shown that the chain of causes cannot be infinite. The First Cause is one and eternal, and is God (cf. Aristot. _Metaph._ 12. 7, and similarly Plato, _Timæus_ 28). Being unchanged this First Cause is perfect, and to know it is the aim of all philosophy, for obviously everything would be intelligible if the cause of all were known. This First Cause is the “necessary being” whose existence is necessary to account for all other existence; it has neither genus, species, nor differentia; it is both external and internal, at once apparent and concealed; it cannot be perceived by any faculty but is knowable by its attributes, and the best approach to knowledge is to know that it is inaccessible. In this treatment al-Farabi is mingling the teaching of philosophy proper with mysticism, in his days rapidly developing in Asiatic Islam, and especially in the Shi`ite community with which he was in contact. From the philosophical point of view God is unknowable but necessary, just as eternity and infinity are unknowable but necessary, because God is above all knowledge: but in another sense God is beneath all knowledge, as the ultimate reality must underline all existing things, and every result is a manifesting of the cause.