Michael Klarman, the Harvard legal historian, has even argued that court cases can create what he calls a backlash. The idea is that some law or legal ruling creates such a stir that people rise up and push things back further away from the initial starting point. In this case the slope is up, not down.

Given how flimsy slippery-slope arguments can be, it is downright scary that they might play an important role in the Supreme Court decision on the new health care law. The case before the court is whether it is constitutional for the federal government to penalize people who fail to buy health insurance.

As everyone concedes, we can’t include the popular rule that forbids insurance companies from discriminating against people with pre-existing conditions unless we encourage nearly everyone to buy health insurance. Although most legal scholars seem to think that the law is constitutional, there is considerable question about whether the Supreme Court will rule that way. And the slippery-slope arguments being used here are just wacky.

Consider these now-famous comments about broccoli from Justice Antonin G. Scalia during the oral arguments. “Everybody has to buy food sooner or later, so you define the market as food,” he said. “Therefore, everybody is in the market. Therefore, you can make people buy broccoli.” Showing remarkable restraint, he did not mention anything about ending up in a roadside ditch.

Justice Scalia is arguing that if the court lets Congress create a mandate to buy health insurance, nothing could stop Congress from passing laws requiring everyone to buy broccoli and to join a gym. He and Chief Justice John G. Roberts Jr. were asking the solicitor general to explain what the principle would be to stop the government from going so far. If the law stands, Justice Roberts suggested, “it seems to me that we can’t say there are limitations on what Congress can do under its commerce power.” He added, “Given the significant deference we accord to Congress in this area, all bets are off, and you could regulate that market in any rational way.”

Please stop! The very fact that a slippery slope is being cited as grounds for declaring the law unconstitutional — despite that “significant deference” usually given to laws passed by Congress — tells you all that you need to know about the argument’s validity. Can anyone imagine Congress passing a broccoli mandate law, much less the court allowing it to take effect?

The irony is that Justices Roberts and Scalia are warning of a risk that they and their colleagues have the power to prevent. Surely, the justices have the conceptual resources to draw a distinction between the health care market and the market for broccoli. And even if they don’t, then all the briefs, the zillions of blog posts and a generation’s worth of economic literature can help them.

More generally, we would be better off as a society if we could collectively agree to ignore all slippery-slope arguments that aren’t accompanied by evidence that said slope exists. If you are opposed to a policy, state your case based on the merits — not on the imagined risk of what else might happen down the road. The path of that road is so unpredictable that it may even produce a U-turn.