Every true crime freak knows that evidence comes in many degrees and dimensions. Evidence can be weak or strong; it can be overwhelming; it can be weighty or flimsy. The metaphors for evidence suggest it works like a physical quantity, like mass. One grain of rice doesn’t have much mass, but a billion has enough to weigh seven tons. Likewise, we all know that circumstantial cases are hard to win (especially without the victim’s body!), but with enough evidence — even circumstantial evidence — it can be hard to resist a guilty verdict. There’s that physical metaphor: resistance, as if evidence exerts force.
As Tom Kelly points out in his SEP article on evidence, what philosophers mean by “evidence” differs a lot from the image suggested in true crime, detective fiction, or science journalism. On Dateline, pieces of evidence include fingerprints on bullet casings or DNA (pesky, pesky DNA). That is, pieces of evidence are chunks of reality, or at least representations of it, as in hearsay evidence. For some philosophers, though, only mental states count as evidence. The fingerprint itself isn’t evidence that Jesse fired the bullet; the evidence is what happens in you when you match Jesse’s prints to the prints on the bullet casing.
All this suggests to me that there might not be a single concept of evidence, and that anyone studying knowledge might be better off not using the term at all. There might not even be a “common sense” notion of evidence, or a common denominator core that’s constant across legal, scientific, and everyday contexts. It’s a real mess out there, folks.
Having said that, I really like all the metaphors in the neighborhood of evidence. They’re very interesting to me, mainly because they use physical notions to evoke normative ideas. And that’s neat! And fraught with danger!
Robert Nozick noticed similar things in how philosophers talk about knockdown arguments. A knockdown argument is so good, so earth-shatteringly convincing, that it bowls you over with its correctness. It lays you out flat. Knockdown arguments beat all comers by technical KO. Nozick’s point in putting this phrase under the microscope was simple: what use do philosophers have for pugilistic imagery? Well, I agree that we’re all friends here, but I just want to note that the forceful imagery reappears in Nozick’s Philosophical Explanations, not as interpersonal violence but as push and pull. “My value fixes what behavior should flow from me; your value fixes which behavior should flow toward you. Value manifests itself as a push and as a pull.” (Phil. Explanations, p. 401)
Arguments can knock us down, and value determines how we should behave kind of like how aquifers and terrain determine the course of rivers…or maybe like how logic gates determine the flow of electricity in a circuit? Reasons, in general, knock us around. They do things to us. Evidence can overwhelm us, drag us kicking and screaming into the light (except in cases of epistemic akrasia). The idea of evidential support fits into this picture. Evidence supports theories or beliefs. Evidence props up some views over others.
All these metaphors suggest that evidence has a sort of power, but particularly a normative power. Evidence transmits believability juice to some views over others. Eddington’s eclipse experiment transmitted evidential impulse to Einstein’s theory of gravity, but not to Newton’s.
Putting it this way is clunky, but that’s my point. If you take the rhetorical simplicity out of the metaphors and make them explicit, the underlying picture looks a little ridiculous. And that’s because the underlying picture really is a little ridiculous. The evidence room at police HQ isn’t just shelf after shelf of normative powers wrapped up in plastic and paper, yearning to break free and confirm various theories.
I think in the end that the metaphors mask something that philosophers are probably right about: the connection between a body of evidence (or justification, or whatever…) and a theory or belief is a connection that arises within human practice. It is not a connection that obtains Out There in the vacuum of logical space. Evidence doesn’t “support” anything unless it supports it for someone.