How was the physical body of the visible universe assembled? Timaios says that fire is necessary for visibility, and earth is necessary for it to be a tangible solid.

However, you cannot create a three-dimensional object with just fire and earth. By placing water and air between the two elements, and connecting them in proportion, the world becomes a harmonious whole.

Dialogue Text

Timaios (Continued): What comes into being must have a body, be visible, and also be tangible. However, if it is separated from fire, it will never become visible, and without some kind of solid substance, it cannot be tangible. And solids cannot exist without earth. Therefore, when the god began to assemble the entire body, he created it from fire and earth. But without a third element, it is impossible to combine the two effectively. For there must be something that gathers them together and binds them. The most beautiful thing among bonds is what can bind itself as well as what it joins with, bringing them as close to unity as possible. And proportion, by its very nature, performs this function in the most beautiful way.

Timaios (Continued) Three numbers, or magnitudes, or some kind of power, in which the middle one has the same relationship to the first as it does to the last, and also if the last has the same relationship to the middle as the middle has to the first, then the middle is both the first and the last, and the last and the first are both the middle. In this way, everything necessarily becomes the same thing, and all things that have become the same are all one.

Timaios (Continued) If the entire body were to exist as a plane, without any depth, then one intermediate term would have been sufficient to connect two things that are linked to it, and also to link itself. However, now the whole should be three-dimensional. And in order to combine three-dimensional elements, one is never enough; always two intermediate terms are necessary. Therefore, God placed water and air between fire and earth, arranging them as much as possible so that fire has the same ratio to air as air does to water, and air has the same ratio to water as water does to earth. In this way, God combined and assembled what is visible and tangible as heaven.

Timaios (Continued). Therefore, from these four things, and by harmonizing the number four in proportion, the body of the cosmos was created. And because it obtained friendship through them, the world came together within itself, becoming a single entity that cannot be separated by anyone other than those who are bound to it.