Are gun manufacturers and distributors liable to people injured or killed by guns? That was the question recently decided by the Supreme Court of Illinois.

Of the two cases that came before the court, one was brought by the city of Chicago, which sought compensation for the expenses related to the illegal use of firearms. The second case was filed by the surviving family members of those who had been killed by crimes involving illegal firearms. The theory of liability – public nuisance and negligence – was the same for both cases.

The city of Chicago offered evidence (based on statistics compiled by the Bureau of Alcohol, Tobacco, Firearms and Explosives) that the accused manufacturers accounted for a greatly disproportionate number of the guns used in crimes. As to the gun dealers sued in the case, the city demonstrated that 1.2% of dealers nationwide accounted for 57% of firearms recovered by federal, state, and local law enforcement agencies. The manufacturers and dealers, plaintiffs argued, knew precisely the use to which their guns were put and designed weapons that were conducive to that use.

Despite this evidence, the Supreme Court of Illinois rejected the two cases. The court based its conclusion on the finding that neither the manufacturers nor dealers had “caused” injury to the plaintiffs.

Causation is the Carrollian rabbit hole of the law, and when a court starts talking about causation, one can be assured that they will be treated to a rare and mystifying discussion.

To a certain extent this is unavoidable. Philosophers, most notably David Hume, have questioned the existence of causation, and it continues to be the source of much debate. But the law assumes that causation exists, and consequently, it appears as an element in nearly every area of the law. The question is, how do the courts determine causation?

The most straightforward test is this: If the harm would not have occurred but for the actions of the plaintiff, then the plaintiff may be said to have caused the injury; if the harm would have occurred in any event, then there is no causation. This is referred to as the “but/for” or “cause-in-fact” test, and it is widely used.

The problem with this test, the courts discovered, is that it includes all sorts of people and entities whom society (or, at least, the courts) were not prepared to hold liable. A company, for example, produces a birth control product that is defective. While the company will be held responsible for injuries resulting from the birth of the child, should the company be liable for a murder committed by that child when he grows up? Under the “but/for” test, the company “caused” the murder.

To avoid this, the courts adopted a second criterion for causation: the defendant must be able to foresee the injury. It would not, to use the above example, be foreseeable that a defective contraceptive device would cause a murder decades later. Therefore, no causation.

This test is referred to as “legal causation,” and it is precisely this test that the Supreme Court of Illinois used to reject the claims of the plaintiffs.

But wait. Didn’t the plaintiffs in Chicago submit evidence that the illegal use of the defendants’ firearms was foreseeable? Yes, but it did not matter, because, as the court conceded, foreseeability is not ultimately about foreseeability but about public policy.

That courts are disguising policy decisions in the language of causation is well known among legal scholars, and the smarter of these have argued that the test of foreseeability should be jettisoned in favor of a more forthright, rational, and predictable measure. Richard Posner has argued that liability should fall upon the person in the best position to avoid the loss at the lowest cost; others would have the courts examine the fairest way of distributing the social risks.

The Illinois Supreme Court gave two reasons for its rejection of the suits against the gun manufacturers and dealers: The public was best served by leaving the decision as to liability to the Illinois legislature; and the suit would encourage actions against a wide range of other companies. Neither rationale is backed up with discussion. No economic, social, or political studies are cited. No expert quoted. Nothing like the focused economic analysis that would have been required under Mr. Posner’s test.

Without this, it is impossible to know how this case should have been decided. Without this, the opposite rationale seems equally plausible: that the courts should impose liability because the Illinois legislature has proved inept at protecting the public from guns; and the public would benefit from a precedent holding liable all companies which manufacture and distribute products intended for illegal use by third parties.

The decision of the Illinois Supreme Court is twaddle. One might as well be reading the Sumerian liturgies, of which it was written that they are “so obscure and remote that one can hardly enter them imaginatively.”

As a longtime hunter, I am disinclined to drive manufacturers and distributors out of business. I am, however, more than happy to close down courts whose decisions are without clarity or intelligence.

Mr. Rips, who was a law clerk to Justice Brennan, practiced law in New York.