This chapter explores both the general working assumptions and definitions which underpin systematic political science and the specific predictive theories which within this overall context constitute a ‘web of explanation’ around the alternation theory of democracy. The developmental party-family theory and the factional theory of party policymaking explain how parties’ identity and ideology shape their choice of policy targets and make them generally unresponsive to other considerations. In the absence of a supporting explanation, theorists are often forced to make a priori assumptions such as these about the ideological rigidity of party behaviour. The theory of government formation interprets party behaviour as driven primarily by ideological and policy concerns rather than seeking office. Policy preferences and their implementation can be classified and measured for over-time and comparative analysis, on the basis of textual, survey, expenditure and other statistical evidence.