It were well, however, to disabuse one's mind of the idea that Newton's work was a finality, that it settled anything. As to why the law of gravitation exists, why bodies tend to come together, the philosopher had little suggestion to offer, and the present generation knows no more than he. Before Copernicus and Newton men looked only with their eyes, and accepted the apparent movements of sun and stars as real. Now, going one step deeper, we look with our brains and see their real movements which underlie appearances. Newton supplied us with the law and rate of the movement--but not its cause. It is toward that cause, that great "Why?" that science has ever since been dimly groping. In the year 1666, when the plague had driven Newton from Cambridge, he was sitting alone in the garden at Woolsthrope, and reflecting on the nature of gravity, that remarkable power which causes all bodies to descend toward the centre of the earth. As this power is not found to suffer any sensible diminution at the greatest distance from the earth's centre to which we can reach--being as powerful at the tops of the highest mountains as at the bottom of the deepest mines--he conceived it highly probable that it must extend much further than was usually supposed. No sooner had this happy conjecture occurred to his mind than he considered what would be the effect of its extending as far as the moon. That her motion must be influenced by such a power he did not for a moment doubt; and a little reflection convinced him that it might be sufficient for retaining that luminary in her orbit round the earth. Though the force of gravity suffers no sensible diminution at those small distances from the earth's centre at which we can place ourselves, yet he thought it very possible that, at the distance of the moon, it might differ much in strength from what it is on the earth. In order to form some estimate of the degree of its diminution, he considered that, if the moon be retained in her orbit by the force of gravity, the primary planets must also be carried round the sun by the same power; and by comparing the periods of the different planets with their distances from the sun he found that, if they were retained in their orbits by any power like gravity, its force must decrease in the duplicate proportion, or as the squares of their distances from the sun. In drawing this conclusion, he supposed the planets to move in orbits perfectly circular, and having the sun in their centre. Having thus obtained the law of the force by which the planets were drawn to the sun, his next object was to ascertain if such a force emanating from the earth, and directed to the moon, was sufficient, when diminished in the duplicate ratio of the distance, to retain her in her orbit.