Carnap's rejection of metaphysics and his embrace of the unity of science are closely intertwined. Carnap is clear about his specific target in metaphysics and about why he rejects it. Surprisingly, on his mature position he does not show us that we cannot be realists, or nominalists, or idealists, etc., but rather how we can. Carnap directs his remarks on the unity of science toward a specific family of claims, prominent in the early twentieth century, namely that the natural sciences are to be sharply divided from the human sciences. Windelband wrote a famous and influential paper that defends such a division. A close look at this paper shows how Carnap's position presents the two-kind-of-science view with a dilemma: Either the attempt to divide the sciences in that particular way fails, or the division crosses the boundary into metaphysics.