Why do circular references and recursion make my program fail?

Question

I wrote this simple Prolog program.

man(socrates).
mortal(X) :- man(X).
immortal(X) :- immortal(X).

I asked it the usual questions, such as

Answer 1

Your definitions - and how you interpret them:

man(socrates).

Socrates is a man.

mortal(X) :- man(X).

Every man is a mortal.

immortal(X) :- immortal(X).

Every immortal is immortal.

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Your definitions - and how Prolog interprets them:

man(socrates).

If you ask about the manhood of Socrates, I know it's true.

mortal(X) :- man(X).

If you ask me about the mortality of someone, I'll check his manhood (and if that's true, so is the mortality).

immortal(X) :- immortal(X).

If you ask me about the immortality of someone, I'll check his immortality. (Do you still wonder how that leads to an infinite loop?)

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If you want to state that someone is immortal if he can't be proven mortal, then you can use:

immortal(X) :- not( mortal(X) ).