"No, mathematical ones! For any quantity—say, position—there is a derivative, representing its rate of change. As I see it, England’s stock of land represents a fixed quantity of wealth. But commerce I see as a derivative—it is the slope, the speed, the rate of change of the nation’s wealth. When commerce stagnates, this rate of change is small, and money founded upon it is worthless. Hence the lopsided exchange rate you told me of. But when commerce thrives, all goes into rapid movement, the derivative jumps up, and money founded on it becomes of much greater value. Once Newton goes to work at the Mint, the supply of coin in England can only improve. Commerce, which has been frozen for lack of money, will surge, at least briefly. The exchange rate between these two currencies will swing the opposite way, long enough at least for me to take a profit."