The “default” way of thinking about evidence is often called foundationalism. Foundationalists think that most of our everyday beliefs about the world are justified by “resting on a foundation” of privileged or more certain beliefs—typically, beliefs about conscious experience, raw feels, or “sense data”. In science, foundationalists typically suppose that a theory in a specialised field is a sort of edifice that is justified by resting on the carefully collected observational “data” of that specific field. This idea is partly inspired by mathematics, in which theorems really do rest on (i.e. are derivable from and implied by) axioms. The question is, should we take mathematics as our model of empirical knowledge?
Opposed to foundationalism is holism. Holists think that everyday beliefs are justified by belonging to a larger belief system. Individual beliefs do not stand or fall on their own, but meet the evidence as a whole, and it’s the way that whole “hangs together” that justifies the entire system. In science, holists typically suppose that theories consist of hypotheses, which are justified by meshing smoothly with other hypotheses, often from disparate fields. This is a matter of how much a theory explains, how reliably it predicts unforeseen observable events, how “virtuous” it seems when we consider its conservatism, modestly, simplicity, generality, fecundity, and so on. This is nearly always an intuitive matter of giving conflicting virtues various weightings, guided by little better than “how it feels” pragmatically.
For example, a holist would judge Freud’s theory by asking how much it seems to explain—how well it meshes with evolutionary theory, with other philosophical ideas about agency, with what ordinary people can see for themselves of undercurrents and tensions in family life, with the various insights that art can give us about ourselves, and much else besides.
A telling difference between foundatonalists and holists is in their respective attitudes to specialist or “expert opinion” (by which I don’t mean the pragmatic know-how of a mechanic, but rather narrow theoretical claims made in advanced disciplines). The foundationalist tends to trust expert opinions, because he sees them as the product of skilled minds’ rare ability to trace specialised claims back to their specialised foundations, rather as an actuary can draw specific conclusions about a company’s finances from its specific account books.
The holist tends to distrust expert opinions. He will remind us that we can more reliably form opinions about the simple, familiar, observable, concrete and everyday than we can about the complicated, unfamiliar, unobservable, abstract or unusual. Most importantly, the holist is aware that claims made in specialised disciplines are typically hypotheses rather than the conclusions of arguments. No “data” implies them. If anything, it’s the other way round: hypotheses imply as-yet unseen events that observation can later confirm or deny. To the holist, the broad experience of a reasonably well-educated layman is better than the specialised training of an expert.
Holism has been around for well over a century. It has some well-known academic proponents such as Quine and Davidson. Yet foundationalism remains the default position among academics. Most of them despise hypotheses—mere “guessing”, as many would put it—and encourage their students to “provide arguments” instead of explaining why this or that hypothesis explains or predicts things better than its rivals. I think this is a tragedy.