Of the separate immaterial entities they affirm, the only one they can establish is the rational soul, for it separates from its body. As for everything else, their method establishes nothing beyond abstractions conceived in minds, namely intelligible universals. Yet they imagine that these exist in the external world, just as the followers of Plato (1) imagine that the Platonic Forms exist in the external world. Accordingly, there would exist (2) universals that are eternal, beginningless, everlasting, and separate (3), such as a universal human being.
This is their error (4): they imagine that what is in minds exists as concrete entities. The same applies to the intellectual substances they posit, of which there are four: intellect, soul, matter, and form. One group among them, such as the followers of Plato (5), posits an intellectual substance called Eternity, another called the Good, and a further intellectual substance—prime matter—which is opposed to form.
Under the most rigorous scrutiny, all these intellectual entities they posit prove to be things conceived within the soul: a person conceives them within himself, so they are intelligibles within his mind, abstracted from their particular instances existing in the external world. The intellect continually abstracts common intelligible universals from specific, observable entities. Thus, it conceives Zayd, Amr, and Bakr, and then conceives a common, universal human being that applies to Zayd, Amr, and Bakr,