Aristotle was Plato’s prize student and studied  under him for twenty years. In sharp contrast to Plato, however, Aristotle preferred careful observation and did not trust purely rational methods Aristotle held that essences existed but could only be discovered by studying nature. His method was to observe, classify, deduce the implications, and then use the deductions as the basis for a new round of observation.

Few ideas were more important to Aristotle than his emphasis on the purposes which our acts are meant to serve. Among the distinctions Aristotle made was that between the potential and the actual. Potential refers to the possibilities open to us. Actual is what we are. We have the opportunity to change and become within the limits of our potentialities.

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Aristotle also reflected on the nature of causality --the "why-because" connection. He distinguished among "material, efficient, formal, and final" causes. The first three explain events in terms of antecedent and concomitant conditions. Aristotle's innovation was to add the fourth, or "final" cause, the end which an act is meant to serve.

In Aristotle's view, our physical nature is composed of matter, but our essence, the soul, is something which gradually comes into being through the course of our development. Unlike Plato, he was not convinced that it survived the body's death. He is said to have distinguished among the "nutritive soul," common to all living beings, the "sensitive soul," common to animals and humans, and the "rational soul," found only in human beings. Each person and each species strives to actualize its own potential. In successive stages of evolution, which is a striving upward toward rationality, more and more potentialities become actual.

Aristotle also collected an immense number of specimens and drew up the basic lines of biological classifications. This one one of the earlier contributions to biology and the development of the phylems.