The naturalising movement in philosophy has its roots in Hume's criticism of cartesian epistemology. Both Hume and Descartes were interested in investigating the processes of the mind. However, Hume came closer to naturalism than Descartes.
Descartes recommended doubting everything as a base for his philosophy. Hume, however, expressed trust in the very mental processes he wished to investigate, arguing that we could not doubt our mental faculties since these were the processes we used to discover how the mind works. Later naturalists also trusted the scientific assumptions which made investigation possible.
Hume emulated the contemporary scientific approach which relied on the empirical confirmation of findings. His epistemology argued that knowledge covered everything we could find out through the use of our reasoning.
Hume thought that some knowledge was the result of causality, which he detected through introspection, rather than reasoning. Despite rejecting conclusions based on cause-effect as beliefs, not facts, he recommended acceptance of information derived from custom as knowledge. He argues that this information must be true knowledge since it is crucial to human survival.
Finally, Hume rejects the total scepticism of the Cartesians, just as he discards total beliefs. He held that it was impossible to generalise both disbeliefs and beliefs.
However, naturalists such as Quine did not follow Hume's introspective empiricism, preferring intersubjective empiricism. Instead of accepting introspective data to account for knowledge, Quine relied on public domain science, the conclusions of scientific discussion. These observations are not introspective, but publicly confirmable.
Quine finally adopts a pragmatic stand on his quest for knowledge:
"Each man is given a scientific heritage plus a continuing barrage of sensory stimulation; and the considerations which guide him in warping his scientific heritage to fit his continuing sensory promptings are, where rational, pragmatic."
Willard Quine (1908-2000) coined the phrase naturalised epistemology in his 1969 paper to describe a philosophical approach to knowledge as a scientific account of the systematic process of understanding the world:
"The business of naturalized epistemology, for me, is an improved understanding of the chains of causation and implication that connect the bombardment of our surfaces, at one extreme, with our scientific output at the other."
He sought to improve the scientific explanation of the relationships between our sensory input and our ideas about the world. The author takes traditional empirical knowledge and interprets it in scientific terms. This requires reformulating the problems of knowledge in the scientific parameters of clarity, evidence and interpretation. He does this by using scientific terminology such as observation and neural intake, instead of the traditional philosophical language of experience and sense data.
Following Russell and Popper, Quine rejects reduction in its appeal to observation as confirmation of a hypothesis. He analyses how we elicit knowledge from our sensory information. He rejects any mental conceptualisation and emphasises that science informs us that our data about the world come from energy hitting our senses and declares that this is the basis for empiricism.
Quine gives an account of how humanity came to learn cognitive language which he says parallels the the way evidence lends support to scientific theory.
"We see, then, a strategy for investigating the relation of evidential support, between observation and scientific theory. We can adopt a genetic approach, studying how theoretical language is learned. For the evidential relation is virtually enacted, it would seem, in the learning. This genetic strategy is attractive because the learning of language goes on in the world and is open to scientific study. It is a strategy for the scientific study of scientific method and evidence."
A cognitive sentence, according to Quine, needs a connection to sentences which confirm sensory stimulation. Language is learned in this way because a child learns to use sentences in response to sensory stimulants. Quine accepts that his approach is behaviourist but clarifies that for him behaviourist means not a conditioned response but a connection to observable evidence:
“What matters, as I see it, is just the insistence upon couching all criteria in observation terms.”
Quite believes that our knowledge of the world is embodied in language and so analysis of language will allow us to to understand the connection between sensory input and our theory of the world. He thinks that children gain knowledge through observation sentences which directly associate meaning and sensory input.
Occasional sentences are true or false at different times and require assent or dissent on each occasion. Observation sentences are a subset of occasional sentences and serve as an objective check of science since they offer the prediction which tests the scientific hypothesis. They require neural input and intersubjectivity as confirmation of objective conclusions.
When a child recognises rain it can utter the words which forms the whole observation sentence: It is raining. Having connected the word with its sensory stimulus the child can then assent or dissent with an occasional sentence when faced with the sensory input of rain. Further learning functions through perceptual similarity which, for Quine:
“...is the basis of all learning, all habit formation, all expectation by induction from past experience; for we are innately disposed to expect similar events to have sequels that are similar to each other."
Intersubjectivity poses another problem. If two individuals perceive a 'rabbit' how can we explain that each, from a different perspective and not sharing the same neural structure, can agree that what they perceive is called a rabbit. Quine explains this by recourse to human perceptual similarity which he affirms must be innate so that perceptual similarity for you is the same as for me. This innateness, he states, is due to natural selection:
"There is survival value in successful induction, successful expectation: it expedites our elusion of predators and our pursuit of prey. Natural selection, then, has favored similarity standards that mesh relatively well with the succession of natural events…It…explains the preestablished harmony: the standards are largely fixed in the genes of the race, the species.”
Intersubjectivity is then partially biologically governed by shared environment. This also implies that the sensory connections between language and the world are biological as well as social. It also means , accordingly to Quine, that we are genetically responsive to more advanced scientific observations.
It is commonly held that an empiricist is someone who believes that all knowledge is based on experience. In the 18th. century, Hume, the example of a classical empiricist, distinguished between knowledge derived from our relations of ideas and empiricism proper. In the 20th. century Russell and Ayer, along with the logical positivists, followed Hume in emphasising the profound difference between mathematical and empirical knowledge. One exception to thus was John Stuart Mill who argued that 2+2=4 is an inductive conclusion from experience since on all past experiences two items brought together with another two items composed four items. However, Mill's theory was criticised as not being revisable and so unempirical.
Quine revived empiricism as a general theory of knowledge and included unrevisability. His starting point is that maths and logic are not freestanding but form part of our web of beliefs. which covers our empirical theories. Our beliefs are all nodes in the web and may be linked through logical beliefs. Some nodes are more central to our worldview, but all form part of the belief web. Quite asserts that we change our web of beliefs by discarding those nearer the periphery and holding on to those in the centre.
The author holds that maths and logic lie at the centre of our web and are deeply bound up with our beliefs since they are essential to our most successful predictive theories. By arguing for the inclusion of mathematical data in a place of our web of beliefs, Quine maintains that there is no profound difference between those beliefs and the indisputable empirical ones.
It was Russell in 1914 then Carnap in 1928 who wanted to demonstrate that, through logic, objects could be translated into statements about sense data. The goal was to show that our supposed knowledge of the external world could be expressed in scientific terms, founded on mathematics and physics. Quine commented unfavourably on this project:
"But why all this creative reconstruction, all this make-believe? 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?"
Traditional epistemology's objective was to include natural science by constructing it from sense data; in Quine's new formulation epistemology is contained in natural science as part of psychology.
In his Meditations Descartes poses the question: "What are the proper starting points for philosophy?” The Cartesian answer was to reconstruct our view of reality from scratch. This approach is what Quine calls 'first philosophy' and he rejects it. Contrary to Descartes, Quine couches his ongoing work in the metaphor of sailors repairing their ship at sea, not in dry dock.
Quite sees no restrictions on the claims philosophers can make. He affirms that philosophy is a continuation of natural science:
"The naturalist epistemologist settles for what he can learn about the strategy, logic, and mechanics by which our elaborate theory of the physical world is in fact projected, or might be, or should be, from just that amorphous neural intake."
In the kantian tradition Ayer believed that science and philosophy were very distinct. He asserted that philosophy does not concern itself with physical properties, but only with their reflections in language.
Quine rejects this account and thinks that there is no difference in type between philosophy and science. He is critical of Ayer's assumption that there exists an analytical-synthetic distinction in which philosophy undertakes to discover the analytical, conceptual truths and empirical science synthetic truths. The author insists that there is no division of labour between science and philosophy. Even armchair philosophy is acceptable to Quine. However, this has advantages and perils. The benefit is that the philosopher can incorporate the supposed findings of other disciplines. The danger is that theories from other systems are only as sound as the theorising.