When we were discussing explanation in Chapter Three, we referred briefly to the role of laws. The statement was that explanations in terms of cause normally rest on one or more general laws, many of them so simple and obvious that it is absurd to cite them. We noted a law of international politics: that deliberate military attacks by one state on the territory of another commonly lead to war. But the reference to law in Chapter Three did not include a definition of the term; nor did it do much to clarify the role of law in the study of politics.
The problem of definition is crucial. The discovery or development of laws of politics can be regarded either as a major or as a very marginal pursuit, depending on the definition that one adopts. When political scientists disagree about the role of law, like as not the disagreement reflects no more than the acceptance of different definitions.
We are concerned here only with descriptive law. To put it negatively, we are not here concerned with the enactment or discovery or development or interpretation of prescriptive law; that is, we are not concerned with the kind of law, whether called jural, moral, ethical, or religious, that is accompanied by an alleged obligation of obedience.1
But what is descriptive law?
Sometimes descriptive law is defined in terms of "invariable association."2 There may be, for example, an invariable sequence between two specified kinds of events; when one of them occurs,