Writing here at Evolution News, Casey Luskin has warned Darwin skeptics against using the line that airily dismisses evolution as “just a theory.” First of all, you’d need to know which of three major definitions of evolution you’re talking about. (See Casey’s post “Is Darwinian Evolution ‘Just a Theory’?“)

Second, the term “theory” is equivocal. In conversation it can refer to mere speculation. In science, we’re always told, it designates something much more firmly grounded. “Theories are neither hunches nor guesses,” writes science reporter Carl Zimmer. “They are the crown jewels of science.”

Casey points out that scientists too sometimes use the word “theory” in the more conversational sense. But leave that aside.

Writing in the NY Times (“In Science, It’s Never ‘Just a Theory’“), Zimmer cites Brown University biologist Kenneth Miller, articulating the familiar talking point about crown jewels. But then he quotes philosopher Peter Godfrey-Smith, who offers a helpful analogy. Says Godfrey-Smith, a theory in the scientific context is like a map. It seeks to relate sets of facts. But then it seems to follow that the map itself is not necessarily a “fact.”

Peter Godfrey-Smith, the author of “Theory and Reality: An Introduction to the Philosophy of Science,” has been thinking about how people can avoid the misunderstanding embedded in the phrase, “It’s only a theory.”

It’s helpful, he argues, to think about theories as being like maps.

“To say something is a map is not to say it’s a hunch,” said Dr. Godfrey-Smith, a professor at the City University of New York and the University of Sydney. “It’s an attempt to represent some territory.”

A theory, likewise, represents a territory of science. Instead of rivers, hills, and towns, the pieces of the territory are facts.

“To call something a map is not to say anything about how good it is,” Dr. Godfrey-Smith added. “There are fantastically good maps where there’s not a shred of doubt about their accuracy. And there are maps that are speculative.”

To judge a map’s quality, we can see how well it guides us through its territory. In a similar way, scientists test out new theories against evidence. Just as many maps have proven to be unreliable, many theories have been cast aside.

Exactly. Zimmer goes on to list “evolution,” however defined, as among those rock-solid theories, on a par with the “general theory of relativity, the theory of plate tectonics, the theory that the sun is at the center of the solar system, and the germ theory of disease.”

And sure, you knew that’s what he was going to say. But the point here is that agreeing that evolution (in its controversial sense of an unguided mechanism successfully accounting for all biological innovation) is a theory doesn’t tell you whether it satisfactorily maps the known facts. It might, or it might not. The question can’t be adjudicated by an appeal to the dictionary, but only by a careful weighing of evidence.

Casey concludes:

How, then, should we speak about “evolution” as a theory? Rather than using imprecise language, and saying things like “Evolution is just a theory,” a better way to express legitimate doubts on the subject is simply to say, “The scientific evidence does not support Darwinian evolution.”

Another way to say the same thing is that as a “theory,” it is a failed — or perhaps more generously, a failing — map.

Image: Peppered moths, by Martinowksy [GFDL or CC-BY-SA-3.0], via Wikimedia Commons.