A fundamental controversy in political theory from ancient times until the present concerns the rationality of political actors, what it is, if it exists at all, and whether or not humans display it in politics. Many political scientists are impatient with this controversy because it remains open after so much (apparently futile) discussion. But they ought not be. The problem of rationality is necessarily imbedded in even the simplest kinds of political research, where, if overlooked, it can occasion misinterpretation and even outright error.
Suppose, for example, in an investigation of legislators one uses the notion of party loyalty as an independent variable to explain behavior. This notion seems simple and straightforward enough and not, therefore, likely to involve one in philosophical controversy. But in fact party loyalty can be interpreted in a variety of ways and the choice among them necessarily involves a choice on one side of the controversy over rationality. Loyalty can be thought of, for example, as a truly independent variable, as a product of political socialization, as an expression of affect, and hence as an essentially irrational motive. On the other hand, it may be thought of as itself dependent on bargains rationally satisfying the preferences of legislators. Such bargains may be either short term or long term so that a legislator's manifest party loyalty may result from a series of advantageous bargains with party leaders on particular bills or from an implied bargain with them on career advantage.