In Timaeus, Plato presents a causal theory of the nature of man. On this account, humans are created by demigods, who take over the soul and the four elements created by the demiurge—the divine craftsman, if you like—and proceed to construct humankind.
Once man has been created he's left to get on with his life. As each individual differs in his innate capacities, not to mention the society in which he lives, human nature will not be uniform but will display noticeable differences depending on both intrinsic and extrinsic factors. Some humans will be godlike, whereas others will be closer to animals.
Timaeus also argues that all forms of life will eventually be reincarnated—that is to say, reborn in a future life. Through the endless cycles of reincarnation life on earth will become diversified, thus extending reason into every aspect of created existence. In other words, the heavens, which are governed by reason, will be brought down to earth.
But reason, though becoming ever more present on earth, will not be distributed equally. This helps to account for the differences between individuals in their capacity to reason and the inability of most of them to grasp the higher truths, unlike philosophers such as Plato and Socrates.