Possibly Dr. Samuel Clark? JA's notes on Ocellus (M/JA/9, Folder 4) mention "Dr. Sam Clark's argument for self-existence." This may be the Samuel Clark, who was author of A Demonstration of the Being and Attributes of God ... Being ... Sermons Preach'd in ... 1704 (2d edn., London, 1706, in MBS, Dowse L. VI. 2).