Throughout much of its post-Reformation history, at least in Western Europe, prominent contributions to philosophy were made almost exclusively by people who also made prominent contributions to the natural sciences, and their contributions to each informed their contributions to the other. But why is this? Is it because the information that we get from the natural sciences is highly relevant to answering philosophical questions? Or is it because the findings of philosophers, such as they are, are highly relevant to answering the empirical questions raised by the natural sciences? In the century-long tradition that began with Gottlob Frege’s seminal writings in the 1870’s, and ran through much of the philosophy written in English in the 1950’s and 1960’s – the tradition that has come to be known as “analytic philosophy” – the prevailing answer to both of these last two questions was: no. According to this analytic tradition, the findings of the natural sciences were of virtually no relevance to philosophy, and vice-versa. Natural science was engaged in the enterprise of constructing a rational, coherent, and true understanding of how the world works, using the materials furnished by sensory experience. Philosophy, in contrast, was engaged in the enterprise of trying to delineate the rules by virtue of which any particular state or event would count as rational, coherent, true, or understanding, at all, and it did so independently of the materials furnished by sensory experience, and relying only on reasoning. Philosophy and natural science were not simply distinct, but neither could supply much useful information to the other.
For instance, in his Logisch-Philosophische Abhandlung, published in 1921, the great Austrian philosopher Ludwig Wittgenstein wrote:
"4.111 Philosophy is not one of the natural sciences.
4.1121 Psychology is no more closely related to philosophy than any other natural science....
4.1122 Darwin's theory has no more to do with philosophy than any other hypothesis in natural science."
The idea present in these passages was one shared by Gottlob Frege, Rudolf Carnap, A.J. Ayer, P.F. Strawson, and other prominent analytic philosophers in the century roughly spanning 1870 to 1970. These analytic philosophers thought of the philosophical enterprise as one of discovery – by means of reflection alone, and without any essential dependence upon empirical information – of those rules by virtue of which something was good or bad, right or wrong, valid or invalid, true or false, rational or irrational.
How might this sort of philosophical enterprise proceed? Gottlob Frege’s work in the foundations of arithmetic provided the paradigm. By the 1870’s, a great deal was known about arithmetic, and Frege, a professor of mathematics at the University of Jena, was quite familiar with this vast body of knowledge. But Frege was interested not so much in extending the body of arithmetical knowledge as he was in discovering what it was, fundamentally, that made all of these known arithmetical facts true, and what it was, fundamentally, that made the proof of an arithmetical claim a valid proof. In order to discover these two things, Frege set about (in his die Grundlagen der Arithmetik, published in 1884) trying to find the smallest and simplest possible set of axioms, and the smallest and simplest set of rules of derivation, such that those rules, when applied to those axioms, would result in proofs of all known arithmetical truths and no known arithmetical falsehoods.
Around 1910, Bertrand Russell and Alfred North Whitehead extended Frege’s efforts, and attempted to axiomatize all of mathematics (in their three-volume Principia Mathematica, the volumes of which were published in 1910, 1912, and 1913). Soon after, Russell attempted something even more ambitious (in his Our Knowledge of the External World from 1914): to find the smallest and simplest set of axioms, and the smallest and simplest set of rules of derivation, such that those rules, when applied to those axioms, would result in proofs of all known truths concerning physical objects. This latter program was most fully executed by Rudolf Carnap in his work Der logische Aufbau der Welt, published in 1928. In this program, natural science was relevant at only one point, and that was in determining which statements concerning physical objects were true, and so needed to be derived within the axiom system that Carnap was attempting to formulate. But once the truths concerning physical objects are fixed, Carnap thought that the information provided by the natural sciences could have no further relevant to philosophy.
One way of understanding the late twentieth-century movement to "naturalize epistemology" is as a reaction against the idea that the natural sciences have such limited relevance to epistemology. Epistemological naturalists take the findings of the natural sciences to be relevant to the epistemological enterprise of discovering those rules by virtue of which a cognitive state is rational or not, knowledge or not, or, more generally, correct or not. Many such epistemologists take the findings of psychology to be so relevant, and some of them also take Darwin's theory to be relevant. But there are many different ways in which one can take the findings of the natural sciences to be relevant to epistemology, and consequently, many different forms of naturalism in epistemology. In this essay, I discuss two very different versions of naturalized epistemology: one version due to W.V. Quine, and the other due to Alvin Goldman. I then mention, very briefly, a few other forms of naturalism in epistemology.