I LaWs and Teories I write this book with three aims in mind: first, to examine theories of inter national politics and approaches to the subject matter that make some claim to being theoretically important; second, to construct a theory of interational pol itics that remedies the defects of present theories; and third, to examine some applications of the theory constructed. The required preliminar to the accom plishment of these tasks is to say what theories are and to state the requirements for testing them. I Students of international pol itics use the ter "theor" freely, often to cover any work that departs from mere descripti on and seldom to refer only to work that meets philosophy-of-science standards. The aims I intend to pursue require that definitions of the key ters theor and law be careful ly chosen. Wereas two definitions of theor vie for acceptance, a simpl e definition of law is widely accepted. Laws establi sh relations between variables, varables being concepts that can take different values. If a, then b, where a stands for one or more independent variables and b st ands for the dependent var able: In for, this is the statement of a law. If the rela tion between a and b is i nvarant, the law is abso lute. If the relation is highly constant, though not invariant, the l aw would re ad like this: If a, then b wi th probabil ity .. A law is based not simply on a relation tha t has been found, but on o ne that has ben found repeatedly . Repeti tion gives rise to the expecta tion that if I find a i n the fut ure, then with speci fi ed probabil ity I will also find b. I n the natural sciences even probabil ist ic laws contai n a strong imputation of necessity. In the social sciences to say that prons of specified income vote Democratic with a cert ain probability is to make a l aw-like st ate ment. The word like i mpl ies a lesser sense of necessi ty . Still, the s tatement would not be at al l like a law unless the relation h ad so often a nd so reliably been found