Vitali, Theodore R.  “Peirceian Influence on Hartshorne's Subjectivism.”  Process Studies 7 (Wint 1977): 238-249.

Abstract

Hartshorne's philosophy is an ontology of subjectivity. The foundational insight is that experience is creative synthesis. Each event of experience is a synthesis of its past, achieving a new unity not entirely reducible to the ingredients of its past. Hartshorne received important influence in the formulation of this theory from Whitehead and Peirce. However, it is contended in the article that the Peirceian influence, especially with regard to fundamental categories and the theory of emergent possibility, is more fundamental than the Whiteheadian. It is suggested that a clearer understanding of Hartshorne's departures from Whitehead can be had if one attempts to read Hartshorne through Peirce rather than Whitehead.  [Abstract from ATLA Religion]