People who favor abortion rights can point to a woman’s right to control her body; people who oppose abortion can point to a fetus’s right to live. People who favor gun rights can point to a gun owner’s right to bear arms; people who favor gun restrictions can point to Americans’ right not to die and be injured by gun violence. The death penalty debate also has fainter echoes of a rights-based argument, on both sides. And no matter how important you may believe action on climate change to be, the argument does not start with self-evident truths about freedom.

It’s true that opponents of same-sex marriage have tried to make rights-based arguments — to say that they are harmed by others’ marriages, just as opponents of interracial marriage did in decades past. But these arguments lacked any evidence that one couple’s marriage was damaged by another’s.

On abortion, by contrast, each side can ground its argument in civil rights. Either the government must deny a woman the ability to control her body or it must deny a fetus life. Yes, the two sides in the debate each believe that one of these denials utterly overwhelms the other. It’s possible one of the two arguments will ultimately prevail. But most Americans have long seen an unavoidable tension between a mother’s rights and a fetus’s.

If you doubt this distinction between the categories, consider the very different paths that public opinion takes on the two sets of issues. On more straightforward civil-rights issues, opinion almost seems to follow the laws of physics: A body in motion tends to stay in motion. Once opinions start to move, they continue doing so.

In the second category, opinions often ebb and flow. Opinion can indeed change radically over time (as it did with, say, Medicare, which was once a hotly debated program that many saw as akin to socialism). But the fact that views start to shift one way doesn’t guarantee that they will continue moving in the same direction.