Nature and essence in Aristotle's logical and biological works

Aristotle holds that some of the predicates that are true of a subject, signify what the subject is. The definition that states what a subject is, is composed out of such predicates, and signifies the essence or nature of the subject. I examine the grounds for distinguishing these predicates in Aristotle's works and I focus on the case of living kinds. In the first part I consider Aristotle's logical works, where the distinction rests on priority relations that relate to explanatory considerations. Given the subjects of a domain or genus studied by a science the predicates that are explanatorily more basic form the essence of those subjects. The aim of scientific understanding is to study the explanatory patterns within a genus; patterns that will be different in different sciences. In the case of the subject-genus of biology the explanatory patterns are determined by teleological relations. In the second part, I consider Aristotle's views on teleological explanation in biology, and in particular what forms the basis of these explanations, thus determining the order of priorities with respect to the nature or essence of the subjects studied in biology