Rufus. From the remarks of Epictetus on friendship
1Such was, and is, and will be, the nature of the universe, and it is not possible for the things that come into being to come into being otherwise than they now do. And not only has mankind participated in this process of change and transformation, and all the other living beings upon earth, but also those which are divine, and, by Zeus, even the four elements, which are changed and transformed upwards and downwards, as earth becomes water, and water air, and air again is transformed into ether; and there is the same kind of transformation also downwards. If a man endeavours to incline his mind to these things, and to persuade himself to accept of his own accord what needs must befall him, he will have a very reasonable and harmonious life.
Source: Stobaeus. IV. 44, 60. Musonius, frag. 42
1 That is, from the heavier to the lighter, and again from the lighter to the heavier.