I do like those two quite a bit, and that probably comes through in the book. There's also a lot to be said for some of the other characters. I like Robert Hooke, who's a real person. I like Daniel Waterhouse, who's fictitious. And some of the people on Leibniz's end of the story are also quite fascinating individuals. Sophie, the electress of Hanover, who was Leibniz's patron, appears to have been a really fascinating and cool woman.