As for their division of the necessary attributes into three classes—one that is intrinsic to the essence, and another that is extrinsic, with the extrinsic itself falling into two subdivisions: (i) what is necessary for the essence and (ii) what is necessary for its existence—and their assertion that the essence is a compound of those intrinsic attributes, the so-called essential ones, namely the genus and the differentia (such as “animal” and “rational”), and that the human being is composed of these two; and their further claim that a genuine definition is valid only when it includes the shared essential attributes constituting the genus together with the distinguishing attributes constituting the differentia, so that the composite species is completed by the two just as man is completed by animality and rationality—this is one of the errors for which the Muslim dialecticians have taken them to task. I have already quoted some of the dialecticians’ remarks on this in other places—when commenting on al-Muḥaṣṣal, on the logical section of al-Ishārāt, and on Greek logic, both in a lengthy monograph and in a shorter treatise, among other works.
The terms “essence” (mahiyyah) and “existence” (wujūd) may be used so that “essence” denotes what is conceived in the mind, whereas “existence” denotes what is found outside the mind. This meaning has never been disputed by the Muslim dialecticians. There is no doubt that the quiddity conceived in the mind is not identical with the existent outside, yet when the mental conception corresponds to what is outside, it constitutes knowledge—just as a statement whose meaning resides in the mind and whose words are uttered by the tongue is true when it matches external reality, although the statement itself is not the very thing spoken of. In the same way, when we form within ourselves a notion of a thing’s quiddity, the mental image corresponds to what exists outside without being identical with it. Therefore, whoever says that essence is other than existence, intending by “essence” the intellectual concept and by “existence” the external reality, is correct. But if he means by both essence and existence together