A proposal to formulate nomothetic theories -- general statements or scientific laws -- in a way to allow for an immediate test of individual differences is presented. The need for such a proposal is cited in recent developments in structural model building in the area of memory called information processing and in the concern to bring individual differences into the main stream of theory construction. It is felt that if a process or mechanism can be measured reliably outside of the situation for which it is serving a theoretical purpose, then an immediate test of the validity of the theoretical formulation is included in the theory. A broad perspective of theory construction is described, showing how the individual differences approach fits into this perspective. Three guidelines for a mechanism to mediate the observed empirical relationships between various independent variables and the key phenomenon of interest are outlined and illustrated. Some objections to the individual differences approach are discussed. The approach is defended for making individual differences variables as a fallout from nomothetic theory construction when such theories deal with fundamental processes of behavior. (Author/KSM)