Willard V. Quine on Naturalism, Science, and Testability"Naturalism; Or, Living within One's Means" in Confessions of a Confirmed Extensionalist (Harvard University Press: 2008; orig. 1995), p. 462.
In science itself I certainly want to include the farthest flights of physics and cosmology, as well as experimental psychology, history, and the social sciences. Also, mathematics, insofar at least as it is applied, for it is indispensable to natural science. What then am I excluding as “some prior philosophy,” and why? Descartes’ dualism between mind and body is called metaphysics, but it could as well be reckoned as science, however false. He even had a causal theory of the interaction of mind and body through the pineal gland. If I saw indirect explanatory benefit in positing sensibilia, possibilia, spirits, a Creator, I would joyfully accord them scientific status too, on a par with such avowedly scientific positions as quarks and black holes. What then have I banned under the name of prior philosophy? ¶ Demarcation is not my purpose. My point in the characterization of naturalism … is just that the most we can reasonably seek in support of an inventory and description of reality is testability of it observable consequences in the time-honored hypothetico-deductive way — whereof more anon. Naturalism need not cast aspersion on irresponsible metaphysics, however deserved, much less on soft sciences or on the speculative reaches of the hard ones, except insofar as a firmer basis is claimed for them than the experimental method itself.