Paul's research has been particularly focussed on the philosophy of Gottfried Wilhelm Leibniz. His publications include The Leibniz-De Volder Correspondence - a critical edition and translation from manuscript sources with an extended introduction (Yale, 2013); Locke and Leibniz on Substance ed. (with Tom Stoneham) (Routledge, 2015); Leibniz and His Correspondents ed. (CUP, 2004); and numerous scholarly about Leibniz.
He is currently working on Leibniz's philosophical theology and ethics, with a particular focus on the foundational role that religious experience plays in these writings and in Leibniz's philosophy as a whole.