Naturalism (philosophers who claim to be naturalists) aims to ally philosophy more closely with science. Essentially reality is exhausted by nature, and all questions including any existential questions (questions pertaining to the human spirit) can be addressed by the scientific method. This is a broad idea and most likely includes the majority of contemporary philosophers. Naturalism simply rejects the supernatural and looks for natural terms to address questions of the human spirit, of which science is a strong contender. Rather than looking for a precise definition of what a naturalist philosophy looks like, it is more useful to rank various philosophies in their degree of naturalist commitment.
Naturalism can be divided up into ontological and methodological kinds. The ontological component is concerned with the contents of reality. The methodological component is concerned with ways of investigating reality, and claims some kind of general authority for the scientific method.
According to Quine (1969), epistomology is concerned with the foundations of science. To show science has an adequate foundation, epistemologists derive statements about the world around us from statements about our own sensations. Given that we are certain about our own sensations (non-skepticism), we can be certain about derived truths about the world. Quine argues that this is a failed approach and recommends what appears to be the abandonment of epistemology all together.
The stimulation of his sensory receptors is all the evidence anybody has had to go on, ultimately, in arriving at his picture of the world. Why not just see how this construction really proceeds? Why not settle for psychology? (Quine, 1969: 75)
Quine’s idea is to abandon the question of whether we have knowledge, and instead study the ways in which we form beliefs: psychological processes.
Epistemology, or something like it, simply falls into place as a chapter of psychology and hence of natural science. It studies a natural phenomenon, viz., a physical human subject. This human subject is accorded a certain experimentally controlled input — certain patterns of irradiation in assorted frequencies, for instance — and in the fullness of time the subject delivers as output a description of the three-dimensional external world and its history. The relation between the meager input and the torrential output is a relation that we are prompted to study for somewhat the same reasons that always prompted epistemology: namely, in order to see how evidence relates to theory, and in what ways one’s theory of nature transcends any available evidence…But a conspicuous difference between old epistemology and the epistemological enterprise in this new psychological setting is that we can now make free use of empirical psychology. (Quine, 1969: 82–3)
This is known as Quinean Replacement Naturalism and finds few supporters in contemporary philosophy. Quine himself toned it down in later texts, accepting the interest and relevance of questions as to the nature of knowledge and justification, and the use of reliable processes and methods that lead to truth. Whether we use such processes and methods itself is a perfectly viable epistemological question.
From the Stanford Encyclopedia:
One recent author who does defend a view close to Quine’s is Hilary Kornblith. Kornblith contends that once traditional epistemologists admit that the Cartesian program of deriving beliefs about the world from certain foundations fails, they end up endorsing as legitimate whatever principles enable them to ratify the beliefs they started with. He writes:
Of course knowledge is possible if we weaken the standards for knowledge far enough, in particular if we weaken them until we can show that many of our beliefs then pass the standards. But this seems to be nothing more than an exercise in self-congratulation. Why should we care about knowledge so defined? (Kornblith, 1999: 160)
He goes on to say:
But if our standards for knowledge are merely designed to allow us to attach the epithet ‘knowledge’ to whatever it is we pretheoretically believe, then … the result is an uncritical endorsement of the epistemological status quo. (Kornblith, 1999: 160)
A grouping of particulars into kinds, of which such ordering does not depend on humans i.e mammals, reptiles, the chemical elements etc.