In Salon today, Michael Lind writes:

…two contenders for the Republican presidential nomination debated whether it is a fact or a theory that humans, chimpanzees, gorillas, orangutans and gibbons descend from a common ancestor.

Lind’s phrasing illustrates the perils of not attending to the distinction between the popular understanding of the word theory and its scientific and philosophical meaning. In the excerpt above, Lind perhaps unwittingly echoes the anti-scientific right in using theory pejoratively, as a term of abuse, even as he attempts to discuss its application to scientific reasoning. Failure to identify and insist on the distinction between the two meanings, whether through ignorance, carelessness, or deliberate conflation, only more deeply entrenches the anti-scientific rhetoric that is so prevalent in American politics.

Theories are attempts to form a coherent and systematic understanding of disparate facts. Facts are inert; theories are useful. Almost anything that you understand or believe is based on some theory or other. Imagine, as social scientists like to do, that you are in a prehistoric jungle. Your companion eats the fruit of a tree and dies. You might sensibly conclude that, if you ate the same fruit, you would also die. But that is just a theory, not a fact. The only fact at our disposal is that the companion died after eating the fruit.

To take a more interesting example, consider gravity. It is hard to imagine any serious contemporary politician disputing the theory of gravity. But gravity is just a theory. To be sure, it is an extremely good theory: it relates such disparate phenomena as the fact that you don’t float off the ground, the fact that the moon and planets follow the particular paths that they do in the night sky, the fact that our communications satellites stay in the positions we expect them to so we can talk to our friends in faraway places while we watch Andy Reid waste timeouts on another coast, and so on. But it is still just a theory; it has, as Rick Perry might put it, some gaps. It is perfectly conceivable that the theory of gravity may at some point be supplanted by a better and more comprehensive theory (indeed, this has been a major project of theoretical physics for decades), and that future generations might look back at us and say, “Can you imagine? They believed in gravity!”

Appreciating the distinction between facts and theories is central to any rational discussion of science, in politics or elsewhere. Lind asks whether the descent of the great apes from a common ancestor is a fact or a theory, but this is a category mistake. The relevant facts include the existence of the various hominids and hominid fossils, the details of their genetic code, and so on. Any hypothesis, positive or negative, about their relationship now or in the past is a theory. Indeed, the conviction that we come from somewhere, that things happened before the start of recorded history, is a theory. It may be a theory with no plausible alternative—i.e., a very elegant and compelling theory—but it is a theory all the same. If journalists and public intellectuals paid more careful attention to the very different senses of the word theory in popular and scientific usage, politicians and others might have more trouble dressing up anti-scientific rhetoric in respectable lexical clothing.