Aristotle had a teleological view of nature. Natural behaviour and natural structure usually have final causes, but these are constrained by necessity: nature does the best she can ‘in the circumstances’. Aristotle’s teleology is sometimes summed up in the slogan ‘Nature does nothing in vain’, and he himself frequently uses similar aphorisms. ‘Nature does nothing in vain’ is a regulative principle for scientific enquiry. ‘Teleology’ asserts that, even though Aristotle knew that some aspects of nature are functionless, he recognized that a grasp of function is crucial to understanding nature. References to the prudence of nature are not childish superstition, but reminders of a central task of the natural scientist.