Thus, the quest to justify natural knowledge by reducing bodies to sensory terms was abandoned. If, then, we cannot justify knowledge on these terms, the best we can do is to explore how knowledge originated and evolved, in the ontological (and phylogenic, if I may take it a step further) sense, and how evidence relates to theory. In favoring psychology over rational reductionism, Quine says, “Better to discover how science in fact developed and learned [sic] than to fabricate a fictitious structure to a similar effect.”
Quine marks the new epistemology as a chapter of psychology, but it seems that, rather than epistemology being subordinate to psychology, they could be mutually supportive of each other. Quine recognizes some may object to this idea, claiming it to be circular, and points out that we are not trying to justify psychology using epistemology, we are trying to understand knowledge. “We are after an understanding of science as an institution or process in the world,” he says, “and we do not intend that understanding to be any better than the science which is its object.”

The new epistemology, Quine says, is also becoming a matter of semantics. A fundamental part of knowledge relies on observation sentences. He defines an observation sentence as a sentence that everyone in a language-speaking community agrees upon. But what is an observation? When I look at the sky, am I observing the photons that hit my color receptors, or am I observing the blueness that results? Quine contends that an observation is whatever is closest to the sensory receptors, notwithstanding consciousness on our part. Observation sentences then, are about bodies rather than impressions, because observations are what we agree on. It doesn’t necessarily matter then, that when we look at the sky I may perceive one version of “blue” and you may perceive another. We both agree that the sky is “blue,” because we are referring to a physical phenomenon outside of ourselves that gives us both some sort of impression, congruent or not.

This account, it seems, is a total naturalization of knowledge. Quine rejects the idea that we have knowledge prior to experience. On the contrary, our observations (and not even ones we are necessarily conscious of) determine our “abstract” knowledge. According to Quine, all of our knowledge comes ultimately from the external world.
Of course, naturalism may imply that our knowledge isn’t the cause of some divine, mysterious force—knowledge is subject to the mechanical inner workings of the brain, which was sculpted unconsciously by evolution, which in essence follows the paths paved by physical law. This naturalization, then, may steer the foundations of knowledge in the direction of a survival mechanism that evolved due to certain environmental factors—a series of fortuitous genetic mutations that thrived and continued to evolve into what we consider knowledge today—and this seems to relegate us to little more than physical systems reacting to our environment. Some would disagree with this cynical look at naturalism and say that knowledge, with all its burdens, is a liberating phenomenon that gives us the reigns to our own lives and a consciousness to human fate. By bearing this phenomenon, we have an obligation to explore, perpetuate, and adapt it, using any means that hint at an epistemological cohesive whole.

集合論
クワインは論理学を古典的な2結合価の一階述語論理、つまり任意の(空でない)議論領域に含まれている真偽に限定した。