Book 2 line 216 (page66)

Another basic principle you need to have a sound (understanding of)
Understanding of: when bodies fall though empty space
Straight down, under their own weight, at a random time and place
They swerve a little. Just enough of a swerve for you to call
It a change of course. Unless inclined to swerve all things would fall
Right through the abyss like drops of rain. There would be no
Collisions, and no atom would meet atom with a blow,
And Nature thus could not have fashioned anything’
Full stop.

He then goes on to discuss that in an empty void all things will move at the same speed so heavier objects would not collide with lighter ones in their way. How extraordinarily prescient this is. Galileo later said the same thing but it took Apollo 15 in August 1971 to prove that in space a hammer and a feather dropped at the same time would land together. You can see a clip of this on You Tube.