This Article is an attempt to show why the effect of ideological preferences in models of the Supreme Court's decisions works the way it does. First, I present an analysis of the expectations justices can form about the results of their decisions. Next, I suggest that for many kinds of cases the conditions for rational choice cannot be met consistently and that, as a consequence, justices use their policy preferences to reach decisions. I argue that the differential effect of attitudinal indicators in empirical studies turns on the type of decisions that are made in particular classes of Supreme Court cases. I then present an examination of Court decisions that supports this assertion. I conclude by discussing ramifications these ideas have for studies of Supreme Court decision making.