The specter of determinism and its implications for moral responsibility acted as a powerful motive on Kant's “critical” investigation of the structure of human reason and the limits of human knowledge. He was convinced that mathematical physics was on the right path and constituted an example that all natural sciences ought to follow. “I assert” – he wrote in 1786 – “that each special discipline concerning nature (besondere Naturlehre) can contain only so much genuine science as it contains mathematics.” But he stoutly opposed the facile opinion that modern physics can yield metaphysical conclusions concerning the subjects of greatest interest for mankind: God, freedom, and immortality. On such matters he “found it necessary to deny knowledge in order to make room for faith” (1787, p. xxx). Kant's faith was a distillation of Christianity. He understood it, however, not as a supernatural gift, but as the natural response of our “theoretical reason” to the living fact of “practical reason”, the cognitive echo of the voice of duty, so to speak.

Pious Christians had voiced qualms about modern natural philosophy since its inception. So Blaise Pascal, after making splendid con-tributions to geometry and physics, wrote c. 1660 about Cartesian mechanicism: “One ought to say in general: ‘;It happens by figure and motion’; for that is true. But to say which, and to compose the machine, is ridiculous, for it is useless and uncertain and wearisome.