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

How about this little program:

 loopy(Y) :- read(X), Z is X+Y, print(Z), nl, loopy(Y).

Your Mr. Prolog would infer, that loopy(Y) has already been called and would fail.