What was used as a model for the creation of the universe? Timaios considered not individual, imperfect beings, but rather a complete being that includes all living things as its parts, to be that model.
Since that archetype is one encompassing everything, the world too must be one. Here, it will be discussed why the universe exists as a single, living whole, and not as multiple or infinite entities.
Dialogue Text
Timaios (Continued): Then, what kind of creature did the one who assembled this world resemble? We must not adopt any creature that belongs to a partial species. For nothing imperfect can ever be beautiful. But let us consider that the world most closely resembles something that contains other creatures, each individually and by species, as parts. The world is visible to living beings, whereas what we are discussing contains within itself all intelligence.
Timaios (Continued) God, among all things that can be grasped by intellect, desired to make this world most similar to the most beautiful and perfect being. Therefore, he assembled the world as a single visible being, containing within itself all other beings of its kind.
Timaios (Continued). Then, did we correctly call it "one heaven"? Or would it be more accurate to say that there are multiple heavens, an infinite number of them? It is one. If the world was created according to a pattern, then it must be so. For everything that exists and contains all living beings that can be grasped by intelligence cannot exist alongside a second thing.
Timaios (Continued) If a second one exists, then another being is needed to surround the second one, and we would have to say that the second one is part of the surrounding being. In that case, it would be more accurate to say that the second one resembles what surrounds it, rather than resembling a complete being. Therefore, the creator did not create two universes or an infinite number of universes, but created this single, unique universe so that it would resemble a complete being. This single, uniquely born heaven exists and will continue to exist.