In Chapter 6,1 discussed the contribution that the writings of Plato and Aristotle made to the development of taxonomy. But we saw that the method of logical division was a method of gaining and systematising knowledge, not an exercise in systematics. We also saw that idealism derives from Plato. In its extreme form, developed in his later writings, “forms” or Ideas - concepts derived from the common feature of many phenomena (such as “beauty”) - not only were real but were the only safe knowledge ἐπιστήμη (episteme). All data acquired directly by the senses were suspect, deceptive, or even illusory and thus could be expressed only as “opinions” (doxa). The parallel with modern taxonomy was to attribute some sort of reality to archetypes or Baupläne as originally developed in the morphology of Geoffroy Saint Hilaire, the Naturphilosophen and Richard Owen. I contrasted this with essentialism, derived from Aristotle 's further development of logical division, which in taxonomy asserts that every taxon has an essence, a character, or series of characters that validate its reality independently of any explanatory theory such as phylogeny.

In order to lay a foundation for what I have to say about the nature of biology in the next and final chapter, I now have to draw on two other realms of discourse that can be traced back to the ancient Greeks. The first involves induction and deduction, the second causality. Plato believed that Ideas could be discovered by a purely rational process, the dialectic.