Symbolism, Imagery, Allegory
The Ride of His Life
Marcus harps a whole lot on transience—the inability of anything in this world to last for any length of time. He consistently urges himself not to get attached to the things of this world. Even children, he says, are like leaves on the wind, blown away as quickly as they blossom on the branch (10.34).
Marcus continues to use imagery from the natural world to talk about the insanely swift movement of created things through time:
There is a river of creation, and time is a violent stream. As soon as one thing comes into sight, it is swept past and another is carried down: it too will be taken on its way. (4.43)
This river moves from the great recycling center in the sky (basically, the cosmos), where the gods keep the "substance" used to form all created things. From there, the river and roars down throughout the earth, where it takes on the dimension of time. It's kind of a like a wicked water flume ride at a cosmic Six Flags, but, you know, more cosmic.
Rollin' Down the River
While this rushing river may be an efficient way of moving materials, it doesn't make the passengers feel secure. Marcus tells himself that the ride is too fast and short for him to be able to attach himself to anything that he sees dashing past him:
Existence is like a river in ceaseless flow, its actions a constant succession of change, its causes innumerable in their variety: scarcely anything stands still, even what is most immediate. (5.23)
This is Marcus's way of telling himself that the things of this world simply aren't worth any emotional investment, since they will inevitably be taken away sooner or later. And according to Marcus, if things don't last, they don't have value:
In this river, then, where there can be no foothold, what should anyone prize of all that races past him? (6.15)
It's a cold way of thinking, but that's Stoicism for you. For Marcus, who has lost children, parents, adoptive parents, his wife, and many friends and colleagues, it's a philosophy of survival. Understanding that the stream will also carry you away, too, makes the brevity of life at least sort of democratic and bearable. At least according to Marcus.