The word means something a little different when applied to human behavior rather than biology (let alone snack foods). When marriage or certain sexual practices are described as “natural,” the word is being strategically deployed as a synonym for “normal” or “traditional,” neither of which carries nearly as much rhetorical weight. “Normal” is by now too obviously soaked in moral bigotry; by comparison, “natural” seems to float high above human squabbling, offering a kind of secular version of what used to be called divine law. Of course, that’s exactly the role that “natural law” played for America’s founding fathers, who invoked nature rather than God as the granter of rights and the arbiter of right and wrong.

“Traditional” marriage might be a more defensible term, but traditional is a much weaker modifier than natural. Tradition changes over time and from culture to culture, and so commands a fraction of the authority of nature, which we think of as timeless and universal, beyond the reach of messy, contested history.

Implicit here is the idea that nature is a repository of abiding moral and ethical values — and that we can say with confidence exactly what those values are. Philosophers often call this the “naturalistic fallacy”: the idea that whatever is (in nature) is what ought to be (in human behavior). But if nature offers a moral standard by which we can measure ourselves, and a set of values to which we should aspire, exactly what sort of values are they? Are they the brutally competitive values of “nature, red in tooth and claw,” in which every individual is out for him- or herself? Or are they the values of cooperation on display in a beehive or ant colony, where the interests of the community trump those of the individual? Opponents of same-sex marriage can find examples of monogamy in the animal kingdom, and yet to do so they need to look past equally compelling examples of animal polygamy as well as increasing evidence of apparent animal homosexuality. And let’s not overlook the dismaying rates of what looks very much like rape in the animal kingdom, or infanticide, or the apparent sadism of your average house cat.

The American Puritans called nature “God’s Second Book,” and they read it for moral guidance, just as we do today. Yet in the same way we can rummage around in the Bible and find textual support for pretty much whatever we want to do or argue, we can ransack nature to justify just about anything. Like the maddening whiteness of Ahab’s whale, nature is an obligingly blank screen on which we can project what we want to see.