that are sound—for this belongs to the study of unsound doctrines that have no governing principle.

Most of what you find these groups extolling as their knowledge, and claiming to be the exclusive preserve of their eminent members, is falsehood with no reality, as we have pointed out previously.

The Eleventh Argument

Their assertion: “An essence is composed of genus and differentia: the genus is the shared component, and the differentia is the distinguishing component.”

They may be answered:

This composition must exist either in the external world or in the mind. If it exists in the external world, there is no universal species there to which this definition applies; there are only perceptible individuals. Each individual possesses an attribute whose counterpart is found in other animals, such as sensation and voluntary movement, and an attribute whose like is not found in other animals, namely rationality. These two attributes coexist in each individual, just as other attributes and substances subsisting in things composed of the attributes placed within them coexist.

If by animality and rationality you mean substance, a human being does not contain two substances, one living and the other rational. Rather, he is a single substance possessing two attributes. If, then,