Anaximander does at one point describe a vortex motion (dine), in which heavier pieces of the Unbounded settle and lighter pieces fly upwards, but this mechanism is meant explain the separation of the cosmos into heavier and lighter components, as well as the rotation of heavenly bodies. It is never connected to the separation of opposites from the Unbounded, nor is it at all clear how it would help to clarify that matter. We are left without any significant clues, therefore, to help us answer the question of how the opposites are related to, and arise from, the Unbounded.
The opposites are not only crucial to Anaximander's answer to the plurality problem, they are also crucial to his maintenance hypothesis. The lawful state of the world, he believes, is one of equilibrium or balance between opposites. Nature itself, through the governance of the Unbounded, maintains this equilibrium through a mechanism he analogizes to human law courts.
The picture is apparently supposed to go like this: The opposites are continually struggling against each other for dominance. In what sense they are struggling is not clear. Most likely, they are transforming into one another, and thereby destroying one another. The Unbounded steps in and restores the proper balance between them, making them "pay penalty and retribution." The cycles of night and day, and of the four seasons, are perfect examples of the forced equilibrium, and probably the examples Anaxagoras himself had in mind.
The Unbounded, then, is the natural manifestation of physical law, imposing a lawfulness upon continually struggling opposites, and thereby maintaining an equilibrium within the cosmos.
The inherent equilibrium in the cosmos is also responsible for supporting the earth. Unlike his teacher Thales', who believed that the earth was supported by water, and his student Anaximenes, who believed that earth floated on a cushion of air, Anaxagoras was the first to maintain that the earth needs no material support. The earth, he claims, is at rest because it is perfectly balanced within the cosmos. The whole cosmos is symmetrical, with the earth right in the middle. There is no reason, therefore, for it to move in any direction, including down.
In proposing this line of reasoning, Anaximander is the first to make use of an important philosophical principle, most closely associated with the great eighteenth century philosopher G.W. Leibniz: the principle of sufficient reason. The principle of sufficient reason states that there is no effect without a cause, or rather, that nothing happens unless there is a reason for it to happen. Because the earth is equidistant from every part of the cosmos, Anaximander argues, there could be no reason for it to move in one direction rather than another. Therefore, it does not move at all.
Anaximander paints an interesting picture of this perfectly symmetrical cosmos: heavenly bodies are in fact wheels of fire, surrounding the earth. We see only small glimpses of these fiery wheels because we are cut off from them by an opaque air. Within this opaque air, though, are small vents, which afford us a view of small pieces of the fiery wheels. An eclipse occurs, on this view, whenever a vent becomes clogged, blocking our view of the sun wheel.
Perhaps the most startling aspect of Anaximander's thought is his proto- evolutionary theory. Anaximander seems to have anticipated Darwin by over two millennia. He observes that human beings have a long childhood during which time they are heavily dependent on others for their survival. Given this long dependence period, he wonders how the first human beings could have survived, since there would have been no one around to care for them. He reasons that human beings must have evolved from a fishlike creature, since fish have no period of dependence. Modern evolutionary theory agrees with Anaximander; in all likelihood the first animals were fishlike creatures.