Science historians often emphasise the mysticism of Johann Kepler, both a Pythagorean and a Christian, but this is not the only thing that distinguishes his philosophy. He was firmly convinced that the Almighty had created the World according to an architectural concept, mathematically conceived, which man could investigate and understand. This belief caused him much speculation. Nevertheless, the exhausting computations and innumerable unsuccessful attempts which led Kepler to the solution of Mars's motion, testify to the great importance he attached to observation. However, Kepler was not a practical astronomer, and he therefore used the immense mass of observations by Tycho, to whom he was an assistant. To Kepler's mind, a theory was valid only if it perfectly agreed with observational data; otherwise, it must be discarded and replaced with a new one. In the numerous models that Kepler attempted to explain the motion of Mars, he often reduced assumptions to a minimum, trying to let the observations "speak". When, again by means of observational data, he finally realized that the only reasonable explanation of Mars's motion was to admit that its orbit was not circular, he did not hesitate to forsake the most sacred axiom of traditional astronomy.