Earlier this month Onyx, a one-year-old black lab, was hit by a car. Hard. The impact shattered his pelvis and mangled his left hind leg. According to authorities in Kern County, Calif., the owners allegedly did what any knuckle-dragging idiot without a lick of moral sense would do: They hacked off the leg themselves without anaesthetic, then left Onyx to suffer in agony. When animal-control authorities rescued the dog, his stump was crawling with maggots.

Fortunately, Onyx has received proper medical care and is doing much better now. But if the story turns your stomach—and well it should—then perhaps it is time to take a second look at the notion of animal rights. That is what Roscoe Bartlett has done. Bartlett, a Maryland congressman, used to experiment on chimpanzees as a physiologist at the Navy School of Aviation. He has since had a change of heart and introduced legislation to outlaw experiments on the great apes and retire the 500 now in captivity to sanctuaries.

Animals having rights is a contentious notion, and there is a strong argument against it: Rights belong to moral agents, and animals lack moral agency. Driven by instinct, they lack the higher-order thinking skills that enable people to choose between courses of action.

But this argument has some weaknesses. First: Some animals, certain primates especially, actually do think rather well. Second: informed consent. Humans can give it, but animals cannot. If one believes, as everyone should, that relationships ought to be delineated by consent as much as possible, then it follows that scientists should experiment only on people.

Third: the marginal-cases argument, which says: What about the senile, the comatose, or the severely mentally retarded? If it is OK to hunt deer because they lack critical thinking skills, then can we hunt children with Down syndrome?

Most sane people would answer, "no." They would say persons with severe mental retardation have a right not to be hunted for sport, even if they can't articulate it themselves. This brings us to the conundrum pointed out by Peter Singer, author of Animal Liberation: Any quality that only human beings have that might provide the basis for their having rights (such as moral agency) will be absent from some human beings—but any quality that all human beings have (such as self-awareness) will be shared by many animals. So either not all people are equal, or people are equal to (some) animals.

To this, philosopher Tibor Machan offers the broken-chair analogy: Some chairs have broken legs, but they "are still chairs, not monkeys or palm trees. Classifications are not something rigid but something reasonable." Rights, he says, belong to the class of reasoning animals, i.e., humans—even if some members of the classification cannot reason. We should attend to what is normal for the species, not specific cases.

That makes sense until you start to pick at it. James Rachels asks us to consider a chimpanzee smart enough to go to college. It makes no sense to say the smart chimp should not be allowed to attend merely because average chimps cannot. (If you find the example ludicrous, substitute "12-year-old boy" for "chimp.") It makes no sense because "it assumes that we should determine how an individual is to be treated, not on the basis of its qualities, but on the basis of other individuals' qualities."

Likewise, Michael Strawser notes in "Animal Rights from an Anarchist/Libertarian Perspective" that if we are going to apply group standards to marginal cases, then we not only should grant rights to the mentally retarded because they belong to a rights-bearing class, we also should impose duties on them, since they belong to a class of beings that has certain duties. For example, we should hold them responsible for their actions, since that is what we normally do with people. So if a mentally retarded man keeps taking money from a purse, we should put him in prison. But of course society doesn't do that—we fit our judgment to the particulars of the case. And properly so. After all: We are concerned with individual rights, not group rights.

But this brings the subject around to another common argument against animal rights: If we take the notion literally, then it does not stop with a cat's right not to be set on fire for the amusement of spectators. A cat also would have a legal claim against a dog that attacked it. But maybe not. Maybe no more than mentally retarded person A would have against mentally retarded person B, were B to hit A in the face. And yet A still has some kind of right not to be hit in the face.

This leads to the conclusion that both humans and animals can have rights without accompanying responsibilities. But isn't that precisely what the notion of a right entails? Rights—especially natural rights—are things we have just by virtue of existing. They don't come with an invoice.

Or maybe it is not so much that animals have rights, as that we have duties toward them. And maybe the distinction between those two things is meaningless in practice. If your dog gets hit by a car, you must see that it gets proper medical attention. That much is perfectly clear—even if the ultimate reason is not.

A. Barton Hinkle is a columnist at the Richmond Times-Dispatch. This article originally appeared at the Richmond Times-Dispatch.