But when Kant was reflecting on the divine ground of nature, he was assailed by doubts. Studying force in nature is one thing, but deducing God’s existence is quite another. Can one identify God in the rigorous and comprehensive way suggested in his third book? Here the ultimate task clashes with ultimate method. Determining the absolute by means of formal demonstration and empirical evidence is too bold. How could Kant be certain that he was not kidding himself?