Monday, October 12, 2009

The Nature of Evidence

For those of you trying to keep track of my flow of thought, here's a recap: I opened a can of worms back in September when I casually announced that there was evidence for evolution. That was followed by a lot of responses, many of which surprised me by their content. I followed up my initial post by explaining my own view of science, which I think is quite reasonable and reflects what scientists actually do. Looking at the responses to that, I felt it was necessary to clarify my doctrinal stance.

Originally for this post I wanted to explain how hierarchies of explanation work, but I thought it would be useful to expound on the nature of evidence first. In my last post, I defined evidence as simply data that corresponds to or is consistent with a theory. I come to this definition from my background in biochemistry and from my ongoing readings in the history of science.

In biochem, we deal with submicroscopic phenomena, atoms, and energy, and the like. It's not as if we can simply get a bigger microscope to settle which of two competing theories is correct. It often takes a lot of ingenuity to devise experiments to figure out an enzymatic mechanism, for example. We end up with competing theories because often the evidence we have is equivocal. Data fit two mutually exclusive theories equally well. So this does not bother me in the least. I'm quite used to looking at different theoretical standpoints and recognizing that there's evidence for both. It's just what we biochemists have to do.

In the history of science, there are often times when interpretation of data are uncertain, and a person could justifiably claim that there was evidence for two mutually exclusive theories. The obvious example would be Copernican vs. Ptolemaic astronomy. Copernicus' trick of switching the sun and earth's location helped explain some observations (like why Mercury and Venus were always observed very close to the sun), but his retaining circular orbits and epicycles did not make his model simpler or more accurate than the Ptolemaic. Furthermore, the Ptolemaic model had on its side the everyday observation of the sun's motion across the sky. Galileo did little to resolve this dispute, and Kepler's proposal of elliptical orbits was not universally accepted. It was not until Newton that the Copernican system (actually a heavily modified version of the Copernican system) really triumphed.

Today, we have no doubt that the earth rotates and moves around the sun, but would it then be fair to say that there's "no evidence for the Ptolemaic system?" I suppose it would be accurate if you mean by that simply that the Ptolemaic system is wrong and contradicted by certain observations that are better explained by later Copernican and Newtonian models. But it seems kind of whiggish and triumphalist to claim that there "is no evidence for Ptolemaic astronomy," as if to say that even astronomers who lived before Copernicus were somehow incompetent or willfully ignoring contradictory evidence. As weird as it may be to hear this, there is a kind of evidence for Ptolemaic astronomy, even if it is very weak and easily explained by a better system.

I can hear the complaints now: "Wood believes the sun goes around the earth." For the record, I do NOT believe the sun goes around the earth. I just think it's harsh and unfair to pre-Copernican astronomers to say that there is no evidence for their system. Likewise, I don't think it's at all fair to say that evolution is without evidence (for an example, see Notch). Whether or not a future Copernicus or Newton comes along to replace evolution with something better depends on us. If creationists content themselves with critiquing evolution, nothing will change (or if it does change, it will not be favorable to creationists). If instead creationists apply themselves to the development of new theories of creation, who knows what might happen?

What kind of new theories are needed? That will be the subject of my next post on hierarchies of explanation. Stay tuned!