A molecular theory of the rate of homogeneous vapor phase nucleation is formulated. The ultimate goal is a theory that contains no ad hoc assumptions or arbitrary parameters having magnitudes that must be assigned in an ad hoc manner. The centerpiece of the theory is a defined cluster denoted as the n/v-Stillinger cluster, a hybrid that combines the original Stillinger cluster and the more recent n/v cluster. The Stillinger component assures that redundancy is avoided in the characterization of the cluster and the n/v component makes the Monte Carlo simulation of the free energy of the cluster relatively simple, and also allows dynamics to weight the importance of a cluster to the nucleation rate process. In the companion paper (paper II), dealing with the application of the theory to argon vapor, it is shown that the avoidance of redundancy is of primary importance to the non ad hoc nature of the theory. The theory provides a standard against which subtle inconsistencies in earlier theories, both molecular and phenomenological, are revealed, and should provide guidance for the development of necessary, useful, future phenomenological theories. Earlier theories are described in sufficient detail to allow a clear discussion of such inconsistencies. This is not just a review, since the illustration of important connections and relationships, not carefully examined in the past, forms one of the major goals of the exposition. A fairly general theory for the nucleation rate is given and some of the parameters entering the theory are discussed. This paper provides the basis for its application to argon vapor in paper II.