 But when [light rays] meet certain other bodies, they are liable to be deflected by them, or weakened, in the same way that the movement of a ball or stone thrown into the air is deflected by the bodies it encounters. For it is very easy to believe that the action or tendency to move (which, I have said, should be taken for light) must in this respect obey the same laws as motion itself. (AT 7: 88–89, CSM 1: 155)