Ford, Lewis S.  "Subjectivity in the Making."  Process Studies 21, no. 1 (Spring 1992): 1-24.

Abstract

By examining how Whitehead came to fashion the concept of 'subjective aim', we can discern the shifting conceptions of subjectivity which underlie the composition of "Process and Reality".  He had early rejected a substance model of subjectivity, and sought its replacement.  Initially the single datum from which concrescence springs provided the being underlying the becoming of the concrescence.  When the original datum was replaced by a plurality of past occasions, the unity was placed in the superjective outcome, but how could this retroactively bestow being upon the concrescence?  Instead of becoming based on being, as former conceptions had assumed, becoming is now conceived as capable of producing being.  Even though only being can causally affect its successors, it must derive its existence from that which has the primary existence, which can only be becoming in all its subjective privacy.  [Abstract from The Philosopher's Index]