Even their own kinsmen—Aristotle and his school, followed later by men such as al-Fārābī and Ibn Sīnā—broke ranks with them, rebutting the earlier Platonists. They said that philosophy had first been raw and was then brought to maturity: its point of departure, they claimed, was the study of natural phenomena; from those concrete realities the mind abstracted the intellectual universal and then imagined it to exist outside the mind.

In this rebuttal they were right; yet they themselves went on to say that these intelligible quiddities do in fact exist externally, side by side with sensible, natural beings. Thus they posited an “intellectual matter” alongside the physical substances, and affirmed a disembodied, universal essence that accompanies concrete particulars. They maintained that essence is other than existence, that the causes of essence are one thing while the causes of existence are another—for, they argued, we grasp the concept of a triangle before we know whether it exists. They also said that this essence is veiled by alien coverings, whether sensory or imaginative.

In their Greek logic—received from their first teacher Aristotle, the author of the doctrines of logic, physics, and metaphysics—they classified the necessary attributes of a subject as follows: (1) essential attributes that are internal to the quiddity and constitute it, and (2) accidental attributes that lie outside the quiddity. The accidental they further subdivided into: (a) what is necessary for the quiddity itself, (b) what is necessary for its existence (once they had distinguished existence from essence), and (c) attributes that merely befall it, whether slowly disappearing or quickly disappearing.

This scheme, and the doctrines they built upon it regarding origination, were rejected by the majority of speculative theologians among the Muslims and others, who have detailed the errors involved elsewhere.

Our aim here is only to point out the source of their mistake. Dividing attributes into necessary and non-necessary is, in itself, correct; it is exactly how Muslim dialecticians proceed when they distinguish between essential properties (lāzim) and incidental ones (‘āriḍ). Thus animality, rationality, and the capacity for laughter are inseparable concomitants of man, whereas being Arab or non-Arab, black or white, young or old, Muslim or unbeliever, are merely incidental.