In Thales' system we saw hints that water was divine; here we get an explicit statement identifying the Unbounded with divinity. The Unbounded, we are told, is divine because it is deathless and indestructible. This can be read in two ways. Either being divine simply consists in being deathless and indestructible, in which case the Unbounded is divine by default, or else being deathless and destructible are simply two symptoms of divinity, and being divine actually consists in something else. What else it could mean to be divine is unclear. Perhaps the Unbounded is divine in the sense of being the ultimate motive force. Certainly motion is an important property of the Unbounded.
In Thales' thought, we saw the emergence of the first Presocratic preoccupation: the physis problem. With Anaximander we see the emergence of the other two: the plurality problem and the maintenance problem. The plurality problem asks how the single physis could have given rise to the multitude of objects that populate our cosmos. The maintenance problem asks how the cosmos manages to remain relatively stable and predictable.
In answer to the question of plurality, Anaximander posits the following picture: The Unbounded moves with eternal motion (because it is alive, remember the important principle from Thales identifying life with motive force). As the Unbounded moves, something separates off from it. From this something, in turn, hot and cold separate off. Hot becomes fire, which then forms the sun and other heavenly bodies. Cold becomes dark mist which then forms earth and air, both of which originally moist but are dried off by fire.
(Though hot and cold get the generation of the cosmos going, they are not the only opposites with importance in Anaximander's system. Wet and dry are also invoked a lot too. Opposites are, without a doubt, the most important forces in the system.)
When reading about these opposites, such as hot and cold, it is natural to wonder whether they are meant to be qualities or substances. Unfortunately, this distinction is post-Socratic, and so the truth is probably nothing so concrete as either of these extremes. It is best, therefore, to simply think of hot and cold (and moist and dry, and all the other opposites) as forces, or agents of physical change. These forces are present in varying degrees at different places in the cosmos at all times.
The next obvious question to ask, of course, is how the opposites are related to the Unbounded. We know that they somehow come out of it, but does that mean that they were originally in it? There are several possibilities, two of which are ventured by Aristotle. The first possibility is that the Unbounded really is a mixture of all opposites, and the second is that the opposites are simply modifications of the Unbounded. If the first reading is correct, then the fact that the Unbounded possesses no qualities of its own seems threatened. In fact, the first reading makes it seem as if there is not a single physis, but rather an infinite number (i.e. every possible opposite), a reading that some have adopted. The second reading is troubling for another reason: if the opposites are not within the Unbounded originally, it is entirely unclear how they ever arise from it. This seems to be the problem that Anaximander's student Anaximenes seized upon.