These are concrete, particular individuals. Our intellect, however, when considering each sense image, realizes that there is something permanent and common to all these individuals . While recognizing that Jane, Sally, John, and Peter are different individuals, the intellect realizes that some characteristics are always present in each and every individual. There is something that makes each of these individuals to be what they are, that is, members of the class of human beings. Such characteristics constitute the nature or essence of the individual.
Human beings are substances with living material bodies who are sentient and capable of rationality.
It is the capability to be rational. If these characteristics are missing, there is no human being. Jane, Sally, John, and Peter share this essence or nature. The intellect recognizes this common nature and puts these individuals into the class of human beings. The intellect also recognizes the differences among these individuals.