The Ninth Argument:

It is true that we know there are shared and distinctive attributes. However, distinguishing between these attributes by treating some as essential constituents of the thing defined and others as necessary concomitants of its essence is an invalid distinction. Rather, all attributes inseparable from the thing defined—in both directions of implication—belong to a single category. There is therefore no difference between differentia and property, nor between genus and common accident.

For the essence composed of these attributes must mean either an essence existing outside the mind, an essence conceived in the mind, or something else. If an essence existing outside the mind is intended, then rationality and laughter in human beings are two inseparable realities peculiar to them. If the essence conceived in the mind is intended, then the mind understands that these two attributes belong uniquely to human beings and not to anything else.

If it is said: Rather, conceiving the essence depends on one of these two attributes: one cannot conceive of a human being in the mind without understanding rationality, whereas understanding laughter follows upon understanding what a human being is. This is what they mean by saying:

“An essential attribute is one without understanding which the essence cannot conceivably be understood, or one upon which the essence depends, both in the mind and in external reality.”