The picture above has attracted tens of thousands of retweets in the past day. Many of those retweets have come from people who argue in favor of the image on the right-hand side. Those people are wrong.
To explain why, it’s easiest to start with this anecdote, which I’d always heard attributed to Abraham Lincoln:
A boy goes up to his father and asks, “Father, how many legs would this calf have, calling the tail a leg?”
“Why five, my son,” says the father.
“No, father, he can not. He would have only four,” replies the son.
“Why, calling the tail a leg, you said, my boy.”
“Ah father! But calling the tail a leg, does not make it so, you know,” says the son.
Turns out Lincoln didn’t say it (at least in a place that would be transcribed), though many abolitionists did. Good for Abe, then. This is a silly story. The father is totally in the right here, and his son is a smartass.
Words mean things. If tails were also called legs, the definition of legs would not be “the things that get stood on.” It would be, rather, “the things that stick out of the torso.” But in our world, legs are actually the things that get stood on, and dogs have four of them.
Many writers, like Jay Hathaway at New York, see the picture above and insist that only the diagram on the right shows the true doggy pants.