Rationality: From AI to Zombies

by Eliezer Yudkowsky

When would someone feel the need to strengthen the argument with the emphatic phrase “by definition”? (E.g. “Humans are vulnerable to hemlock by definition!”) Why, when the inferred characteristic has been called into doubt—Socrates has been seen consulting herbologists—and so the speaker feels the need to tighten the vise of logic. So when you see “by definition” used like this, it usually means: “Forget what you’ve heard about Socrates consulting herbologists—humans, by definition, are mortal!” People feel the need to squeeze the argument onto a single course by saying “Any P, by definition, has property Q!,” on exactly those occasions when they see, and prefer to dismiss out of hand, additional arguments that call into doubt the default inference based on clustering. So too with the argument “X, by definition, is a Y!” E.g., “Atheists believe that God doesn’t exist; therefore atheists have beliefs about God, because a negative belief is still a belief; therefore atheism asserts answers to theological questions; therefore atheism is, by definition, a religion.” You wouldn’t feel the need to say, “Hinduism, by definition, is a religion!” because, well, of course Hinduism is a religion. It’s not just a religion “by definition,” it’s, like, an actual religion.

What leads us to argue "by definition"?