“has often been suggested that moral (or practical) reason is distinguished by the fact that it is prescriptive, while theoretical (or speculative) reason is descriptive. That is certainly not right. Moral reason has a vast stake in description. It describes particular things, describes their relations and purposes, describes the way the world as a whole fits together. Without this descriptive exercise practical reason would not be reason at all. It cannot be that “reason is the slave of the passions.”5 That is to say, it cannot be that practical reason begins with a simple impulse, an undetermined will, which then calls on knowledge of what is true and false, independently arrived at, to shape the execution of its project. For the impulse on its own, apart from any rational description, can have no clear project. It cannot be the impulse it is — fear, desire, sympathy, or anything else — unless it knows something about the world from the start: there are things that pose a danger to existence, there is good that offers it fulfillment, there are fellow-beings whose case is like mine. World-description belongs, as they say, “on the ground-floor” of practical reason. There can be no prescription without it; neither can there be description which is neutral in its prescriptive implications. Only because this is so, can we think our way through the world practically.”
―
Oliver O'Donovan,
Self, World, and Time:
