ABSTRACT

The planets move round the Sun only because there is some motive force which causes them to move. Now, what is this force? This is a particularly difficult question, which is made all the more difficult seeing that the location of this force in the Sun prevents any elucidation of its nature; and this location is not absolutely certain. In fact, the case of the Moon provides an objection by showing that the Earth is endowed with a similar force; so, we are compelled not to associate it too closely with the Sun.