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Philosophy professor Stephen Heaney:

Abraham Lincoln once asked how many legs a dog has if we call a tail a leg. The answer, he said, is four: calling a tail a leg does not make it so. We chuckle and move on.

But what if people began to argue that a tail really is a leg? They might say that what defines the leg is that it is an appendage of the dog’s body, that it contains bone and muscle covered with skin and fur—just like a tail. Tails just happen to come out of the body at a different angle than other legs. When a tail hangs down low, who can tell the difference?

This is an example of defining a thing according to non-essential characteristics. It is like saying that a soldier is “a man who wears a uniform and carries a gun,” or calling a football stadium “a field surrounded by lots of seats.” It may be true in each case, but fails to tell the story.

To continue the figure, the bones and muscle of a leg are different from the tail. They have to support the dog and make it possible to run and jump. No matter how well the dog can wag its tail, it will not propel it anywhere. The issue, then, is not that the leg has bones and muscles, but how they are put together, and why. A tail is not a leg, because it is impossible for it to function as one.

Some may respond that there are legs on many dogs that cannot propel the dog anywhere. They have broken bones, or withered muscle, or have lost the foot in an accident. If not all legs can propel the animal forward, then this ability is not an essential characteristic of a leg. If lame legs are legs, so is a tail.

But a wounded leg is still a leg. Repair it, and it will function as one. If it cannot be repaired, this fact does not change the kind of thing it is. It is a leg, though damaged. The tail remains a tail.

Professor Heaney argues that “the call for same-sex marriage involves a similar misdefinition.” To see how he uses this logic to look at the essential characteristics of a marriage, read the rest.

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