5 Rebirth of Aristotelianism in the Philosophy of Science
These problems helped to motivate the first serious reintroduction of the notion of real
essences and necessary properties into philosophy since the work of Leibniz. Crucial to this
reintroduction was the development of a formal modal logic of necessity and possibility by
C. I. Lewis and Ruth Barcan Marcus,29 which gave precision and power to modal reasoning
and the idea of modal properties. Saul Kripke, who developed a semantics for
modal logic, led this metaphysical revival.30 Part of the appeal of Hume’s banishing
necessity from science arose from the unpalatable nature of the position he and the other
British Empiricists were opposing. The rationalist view of Descartes and Leibniz claimed
that knowledge of the necessary features of the world was given a priori, through reflection
upon innate ideas. If a scientific understanding of the world is to include necessary features
of things, though, this means that scientific knowledge can be arrived at through simple
reflection, without the need for experiment. Descartes and Leibniz said just that, but the
Empiricists quite reasonably found this impossible to swallow. Kripke effectively attacked
the view that knowledge of necessary truth need be a priori, while knowledge of contingent
truths must be a posteriori; in doing so, he removed a fundamental prop of the
Humeian position. He also argued for existence of necessary properties in things, and for a
notion of real essences as the properties of things and kinds that science reveals as
underlying and explaining their observable characteristics. Hilary Putnam, whose concerns
were more focused on the philosophy of science and the problems of giving an account of
theoretical terms, also defended the notion of natural kinds, and gave an account of the
reference of natural kind terms that paralleled Kripke’s account of the reference of proper
names.31 The attempt to argue for essentialism on the basis of Putnam or Kripke’s accounts
of reference has not been successful,32 but current neo-Aristotelians do not base their views
on versions of Kripke or Putnam’s direct theory of reference. Molnar, a leading neo-
Aristotelian, is content to adopt the view of ‘good old, much maligned John Locke’ on
nominal and real essence in explaining natural kind terms. (Molnar 2003, p. 22.)