 Kant retired from teaching in 1796. For nearly two decades he had lived a highly disciplined life focused primarily on completing his philosophical system, which began to take definite shape in his mind only in middle age. After retiring he came to believe that there was a gap in this system separating the metaphysical foundations of natural science from physics itself, and he set out to close this gap in a series of notes that postulate the existence of an ether or caloric matter. These notes, known as the Opus Postumum, remained unfinished and unpublished in Kant’s lifetime, and scholars disagree on their significance and relation to his earlier work. It is clear, however, that some of these late notes show unmistakable signs of Kant’s mental decline, which became tragically precipitous around 1800. Kant died February 12, 1804, just short of his eightieth birthday.