[How is it that the elements are perpetually moving?—i. e. How is absence of uniformity, the condition of motion, secured for them? We have seen that there is a continual tendency to produce uniformity, due to the motion of the receiving vessel. There is also a tendency to destroy it, due to the revolution of the universe, which trusts the elements into each other.] Unless a person comes to an understanding about the nature and conditions of rest and motion, he will meet with many difficulties in the discussion which follows.