Long familiarity has made these wonderful laws of Kepler seem such a matter of course that it is difficult now to appreciate them at their full value.

Yet, as has been already pointed out, it was the knowledge of these marvellously simple relations between the planetary orbits that laid the foundation for the Newtonian law of universal gravitation.

Contemporary judgment could not, of course, anticipate this culmination of a later generation.

What it could understand was that the first law of Kepler attacked one of the most time-honored of metaphysical conceptions--namely, the Aristotelian idea that the circle is the perfect figure, and hence that the planetary orbits must be circular.

Not even Copernicus had doubted the validity of this assumption.