In the _Timaeus_ Plato asserts that nature is made of fire and earth with air and water as intermediate between them, so that 'as fire is to air so is air to water, and as air is to water so is water to earth.' He also suggests a molecular hypothesis for these four elements.

In this hypothesis everything depends on the shape of the atoms; for earth it is cubical and for fire it is pyramidal.

To-day physicists are again discussing the structure of the atom, and its shape is no slight factor in that structure.

Plato's guesses read much more fantastically than does Aristotle's systematic analysis; but in some ways they are more valuable.

The main outline of his ideas is comparable with that of modern science.