The chapter is a defense of the thesis that rational agents must desire and act sub specie boni or under the guise of the good. Our thought is that what underlies the guise of the good thesis is a more general point about the explanation of any self-movement, a point that applies, in different forms, to the explanation of behavior in nonrational animals, and even to the explanation of the nutrition, growth, and reproduction of nonsentient living things. What these various kinds of explanation have in common, we suggest, is that they all have a teleological structure; and we argue that, in general, this sort of explanation works by connecting what a creature is doing with what is good for creatures of its kind. The special feature of the application of this explanatory structure to rational creatures is that such creatures belong to a kind in which this connection between action and goodness becomes self-conscious: They are creatures whose action is expressive of and explained by their conception of their own good. This, we argue, is why rational self movers must act under the guise of the good.