Newton's exposition arose from an investigation of the cause of the motion of the planets round the sun, and this in due course led to the enunciation and establishment of the Newtonian theory of attraction. The origin of this theory has been often told, but will bear repetition. The fundamental idea occurred to Newton in 1665 or 1666, shortly after he had taken his degree at Cambridge, when, as he wrote later, "I was in the prime, of my age for invention, and minded Mathematicks and Philosophy more than at any time since." His reasoning was as follows. He knew that gravity extended to the highest hills, he saw no reason why it should cease to act at greater heights, accordingly he believed that it would be found in operation as far as the moon, and he suspected that it might be the force which retained that body in its path round the earth. This hypothesis he verified thus. If a stone is allowed to fall near the surface of the earth, the attraction of the earth causes it to move through sixteen feet in one second: also Kepler's Laws, if accurate and applicable, involve the conclusion that the attraction of the earth on a distant body varies inversely as the square of its distance from the earth. Now the radius of the earth and the distance of the moon were known to Newton, and therefore, on this hypothesis, he could find the magnitude of the earth's attraction on the moon. Further, assuming that the moon moved in a circle, he could calculate the force required to retain it in its orbit. At this time his estimate of the radius of the earth was inaccurate, and, when he made the calculation, he found that this force was rather greater than the earth's attraction on the moon. The discrepancy did not shake his faith in his theory, but he conjectured that the moon's motion was also affected by other influences, such for example, as the effect of a resisting medium which might itself be in motion as supposed by Descartes in his hypothetical vortices.