People are like fish swimming in the long river of time. Lifespan is the length of the fish (affecting time), power is the width of the fish (affecting scope), and belief—or the ability to stir karma—is the height of the fish (affecting depth).
The boundaries between these three are not strictly defined; the metaphor is simply a way to make them clearer.
If a fish is long-lived and powerful but incapable of stirring karma, it merely hugs the riverbed, unknown to the world. It cannot leap above the surface; it cannot affect the big picture. The river remains calm, and its presence or absence makes no difference to the world.
Chapter 657