DI' is a set of natural numbers, PI'(x, y)= 1 if and only if x.

Then' I' is the model of A, and A also has a model.

Take any explanation I that satisfies statement A, and take d 1∈DI, because I (? x？ YP(x, y))= 1, so there is d2∈DI to make PI(d 1, d2)= 1, and because I (? x？ P(x, x))= 1, so d 1≠d2. Because I (? x？ YP(x, y))= 1, so d3∈DI makes PI(d2, d3)= 1, and because I (? x？ P(x, x))= 1, so d3≠d2. Because I (? x？ Y (p (x, y) ∧ p (y, z) → p (x, z)) = 1, so PI(d 1, d3)= 1, so D3 ≠ d/kloc-0. So d 1, d2 and d3 are three different elements in the universe. This process can go on forever and get d 1, d2, d3. Therefore, there must be infinite elements in the universe.