There are three kinds of people in the world;
those who can count and those who can't.

(A similar one:}

There are two groups of people in the world;
those who believe that the world can be
divided into two groups of people,
and those who don't.

(Another similar one:}

There are two groups of people in the world:
Those who can be categorized into one of two
groups of people, and those who can't.

(And then there's:)

The world is divided into two classes:
        people who say "The world is divided into two classes",
    and people who say
        The world is divided into two classes:
                people who say: "The world is divided into two classes",
            and people who say:
                The world is divided into two classes:
                        people who say ...