It is possible for one scholar to treat the attribute as a distinguishing essential, while another may regard it as an inseparable accident of the quiddity.

Ninth objection:
Knowledge of the essence depends on knowledge of the essential attributes—and vice-versa—which necessarily produces a vicious circle.

That is, one may say: this instructional method is circular a priori and therefore untenable. They argue that the entity to be defined cannot be conceived, nor can it receive a real definition, except through the mention of its essential attributes. They then assert that an attribute is “essential” when the quiddity cannot be conceived without conceiving that attribute. Thus they differentiate the essential from the non-essential by declaring: the essential is that upon which the conception of the quiddity rests, so it must be grasped beforehand.

At other times they maintain that the attribute must be conceived along with the quiddity, such that its conception cannot, in their view, be posterior to the conception of the essence; and by that criterion one knows it to be an essential attribute.

In reality, their claim means that the essential cannot be distinguished from the non-essential until the quiddity is known, yet the quiddity itself cannot be known until its essential attributes—out of which it is composed—are known. That is circular.

If the student cannot conceive of the thing defined until he first conceives of its essential attributes, and he cannot know an attribute to be essential until he conceives of the subject—that same thing—then he will not recognize it as essential unless he conceives of the subject, and he cannot conceive of the subject until he has conceived of the essential attributes and distinguished them from the other elements included within it or constituting its parts; for conceiving something as a part of another without conceiving that other is impossible.

We, however, do not claim that an essential predicate cannot be conceived until its subject is conceived. A person conceives of many things without realizing that they are attributes of something else or parts of it, let alone that they are—