"...en Angleterre, ce sont les deux chambres & le roi" — Diderot
Before the publication of the Federalist papers, the clearest statement about the advantages that a unitary executive derives from "secrecy and dispatch" comes from Histoire des deux Indes (xix.2, paragraph 82, p. 78) [see]. But who penned that paragraph? Denis Diderot or Guillaume-Thomas Raynal? With the help of GoogleBooks, I have searched previous works of Raynal, such as Histoire du Parlement d'Angleterre and Histoire du Stathoudérat, and I found no traces of secret, célérité.
* * *
I am pretty sure that the phrase comes from translations of Polybius's Histories. We find it in works by David Hume, Jean-Louis De Lolme, and others (*). But now look at Encyclopédie's article LÉGISLATEUR (1751), by Diderot: "...& c’est ici le triomphe du gouvernement monarchique ; c’est dans la guerre sur-tout qu’il a de l’avantage sur le gouvernement républicain ; il a pour lui le secret, l’union, la célérité, point d’opposition, point de lenteur." Diderot seems to be the only author —before the Federalist papers,— who mentions these magic words in the context of the executive power.
So it's Diderot!
It's time for some conclusions:
(1) Sources. Is Histoire des deux Indes a key source of the Federalist papers? The more I think about it, the more it seems to make sense. A good part of Madison's No. 10 seems to be based on Diderot's chapters 35 and 42 of Book xviii of HDI. (I presented that argument in my book). The same could be said of Hamilton's No. 70.
(2) Mixed government. As the editors of The Founders' Constitution put it, the single executive power is the point at which the old Greek theory of mixed governent and the modern theory of the separation of powers meet. Brilliant!
(3) Political thought. There is no such thing as a "pure" American, English or French political thought. Authors borrowed extensively from each other; books, pamphlets and letters crossed the Channel and the Atlantic in the same vessels that transported goods and ... human beings.
(4) Greek authors. In the end, the top Enlightenment thinkers were the ones that mastered their Latin and Greek sources; among other things, it gave them a key advantage in terms of the felicity of their rhetorical expression. Quick! Time to perform my exercises from Lesson four of 40 Leçons pour découvrir le grec ancien:
O κὀσµoς εστi µακρóς, etc.
(*) David Hume: "...such was the resolution, secrecy and dispatch with which he conducted this enteprise" (History of England) [see]; John Marshall: "....notwithstanding the secrecy and dispatch that were used..." [see]; Guillaume-Thomas Raynal: "L'exécution de ce projet exigeoit une grande célérité, un secret impénétrable..." (HDI 1780, vi.10); De Lolme: "...and military Messengers were sent with every circumstance of secrecy and dispatch..." [see].