SECT.  LXXX.  The Rules of Motion, which the Epicureans suppose do not render it essential to Bodies.

I may be answered that, according to the rules of motion among bodies, one ought to shake or move another.  But where are those laws of motion written and recorded?  Who both made them and rendered them so inviolable?  They do not belong to the essence of bodies, for we can conceive bodies at rest; and we even conceive bodies that would not communicate their motion to others unless these rules, with whose original we are unacquainted, subjected them to it.  Whence comes this, as it were, arbitrary government of motion over all bodies?  Whence proceed laws so ingenious, so just, so well adapted one to the other, that the least alteration of or deviation from which would, on a sudden, overturn and destroy all the excellent order we admire in the universe?  A body being entirely distinct from another, is in its nature absolutely independent from it in all respects.  Whence it follows that it should not receive anything from it, or be susceptible of any of its impressions.  The modifications of a body imply no necessary reason to modify in the same manner another body, whose being is entirely independent from the being of the first.  It is to no purpose to allege that the most solid and most heavy bodies carry or force away those that are less big and less solid; and that, according to this rule, a great leaden ball ought to move a great ball of ivory.  We do not speak of the fact; we only inquire into the cause of it.  The fact is certain, and therefore the cause ought likewise to be certain and precise.  Let us look for it without any manner of prepossession or prejudice.  What is the reason that a great body carries off a little one?  The thing might as naturally happen quite otherwise; for it might as well happen that the most solid body should never move any other body—that is to say, motion might be incommunicable.  Nothing but custom obliges us to suppose that Nature ought to act as it does.