What they are dealing with is universal only in the mind, not in external reality. Therefore, if this is, in their estimation, the supreme science, then their highest science is not knowledge of anything that actually exists outside the mind. Rather, it is knowledge of a property common to all existents, namely the notion of “existence.” The same is true of such general terms as “thing,” “essence,” “reality,” “soul,” “entity,” “quiddity,” and the like. It is obvious that knowing such notions is not knowledge of anything existent outside the mind—neither the Creator nor the creation. It is merely knowledge of a universal, shared concept that exists only in thought. Indeed, some conceptual notions are shared by what exists and what does not, as when we say “that which is mentioned,” “that which is known,” or “that which is spoken of”; these notions are even broader than the former.

This stands in stark contrast to the supreme science recognized by Muslims, namely the knowledge of God. God, in His very Being, is superior to all else in every respect, and knowledge of Him is, from every perspective, the loftiest of all kinds of knowledge and the foundation of every other science. Muslims hold that when knowledge of Him is attained in its fullest sense, it necessarily entails knowledge of everything that exists.

Knowledge of the bare term “existence,” by contrast, has no concrete reality outside the mind. Nor does grasping that common denominator lead to knowledge of its genera, its species, or the distinctive traits of each particular thing. It amounts only to an awareness of a shared attribute that has no external counterpart; it is nothing more than insight into such universals.

Mere acquaintance with these abstractions does not perfect the soul—nor does the study of their general divisions. Suppose we are told that existence divides into substance and accident, and that substance itself comprises five classes, as they claim. This claim is incorrect: of the five, only “body” has actual existence. As for “matter,” “form,” “soul,” and “intellect,” none of these has any reality outside the mind unless it is itself a substance or an accident. What they affirm therefore reduces to a mental construct rather than an external reality, as has been demonstrated in detail elsewhere.

Even some of their own supporters and admirers have admitted as much—for instance, Abū Muḥammad Ibn Ḥazm and others—and because of his high regard