Some philosophers claim that propositions are simple—i.e., lack parts. In this paper, I argue that this claim is mistaken. I start with the widely accepted claim that propositions are the objects of beliefs. Then I argue that the objects of beliefs have parts. Thus, I conclude that propositions are not simple. My argument for the claim that the objects of beliefs have parts derives from the fact that beliefs are productive and systematic. This fact lurks in the background of debates about the metaphysics of propositions. But its import for these debates has yet to be fully appreciated. So here I bring the point to the fore, and thus make manifest a powerful argument against simple propositions.