A view of evidence

In the last month I've discussed various aspects of the evidence-based debate. I'd now like to present some of my ideas on the subject. As I've suggested previously, the definition of evidence seems to lie at the heart of the dispute. But before I attempt to define evidence, I think it's useful to consider the word itself.

Evidence is the noun form of the adjective evident, ultimately deriving from the Latin evidens from ex- (out, forth) + videre (to see). The Oxford English Dictionary gives the following definitions for evident: "1. a. Conspicuous b. Obvious to the sight. 2. Clear to the understanding or the judgement; obvious, plain. 3. Indubitable, certain, conclusive. —1653." And the primary definition of evidence is simply "The quality or condition of being evident."

So a very literal interpretation would take evidence to be what can be seen, or "shown forth". It seems natural to extend this beyond vision to sense perceptions in general. Further metaphorical extensions bring us to the notion of clarity, certainty, and conclusiveness. And already the problem becomes apparent: we have moved from sense perceptions to things that might be considered similarly certain and conclusive. Like what? Deductive arguments? Accepted theory? Long experience? Expert opinions? Religious precepts?

Let's consider each of these in turn, starting with deductive arguments. A valid deductive argument is water-tight—provided that its premises are true. But how do we establish them? Perhaps we can depend on accepted theory. But history is littered with theories that were once universally accepted, but are now discredited or superseded. Of course there's always long experience. Experience is the cumulative product of personal practice and observation. But it is notoriously subject to selection bias (and perhaps other biases too). And when it comes to rare events, no amount of experience is sufficient. Can expert opinion step into the breach? The opinion of an expert represents a synthesis of many different sources of information, usually carried out over many years. While there may be many good reasons to trust an expert (such as his or her qualifications, intelligence, experience, and good standing in the community), well-meaning experts have been spectacularly wrong any number of times. Ultimately trusting expert opinion is an act of faith. Which nicely brings us to religious precepts as a source of certainty. These are perhaps the original "self-evident" truths—that is, to the believer, but perhaps to nobody else.

It is, of course, true that sense perceptions can be misleading too. A classic example is the straight stick that appears bent when placed in a glass of water due to the refraction of light. Any number of other illusions and hallucinations make interpretation of sense perceptions a thorny philosophical problem—one of the fundamental challenges in epistemology. Without wishing to minimize these, I will set them aside, by simply asserting that we know the external world through our sense perceptions. They are the only raw materials at our disposal, and they are the closest we can get to certainty about the external world. Evidence is observational.

But that doesn't mean that observation is all there is to evidence. For instance, a scientific experiment involves action (manipulating or controlling conditions) as well as observation. But if there's no observation, there's no evidence. To consider a particular example, this means that opinion doesn't count as evidence, but observation of opinion does! That is:

• Opinion about X isn't evidence about X.

But

• Observation of opinion about X is evidence about opinion about X.

I certainly don't mean to imply that opinion is worthless—that would be a bit self-defeating, now wouldn't it?—but it just can't be counted as evidence. In any case, regardless of what I or anyone else has to say, opinion will always guide us.

Ultimately, I think that opinion is most convincing when it is backed up—whether by deductive argument, theory, experience, evidence, or some combination of these. And I hope that my opinions have some of these supports.

While I haven't yet given a definition of evidence, I have presented what I think is a crucial qualification: evidence is observational. But there's a lot more to it than that!