The Whitehead quote was gleaned from one of his biographers who remembered the basics of it from conversations with a friend, and has been repeated in various forms. My Master's thesis was to be on Whitehead's metaphysics in "Process and Reality". In it he talks about the difficulty of reducing 'sensa' ("eternal objects") to simple components. Of course, his treatment of the concept(s) is anything but simple.
   In an essay by W. Mays, in a collection by Ivor Leclerc, Whitehead's notion is compared to Occam's Razor.... thus, the simple form of the quote I have always preferred was NOT a direct quote from a book. I can't right now locate the paperback where the friend commented on the idea, but it's the best I can do.
(Gee I hadn't looked into those books for years!)