Ancient Greeks struggled to understand the nature of matter
Empedocles (around 490 to 444 BC) thought there were four original elements: Earth, Air, Fire, Water. He thought everything else came about through their combination and/or separation by the two opposite principles of Love and Strife.
Leucippus (around 460 to 420 BC) and Democritus (around 460 to 370 BC), supposedly a pupil of Leucippus, are considered the founders of atomism. Leucippus regarded atoms as imperceptible, individual particles that differ only in shape and position.
Plato (about 427 to 347 BC) in his work, the Timaeus, proposes a mathematical construction of the elements - earth, air, fire, water. Each of these elements is said to consist of particles or primary bodies. Each particle is a regular geometrical solid- the cube, tetrahedron, octahedron and icosahedron. Each of these particles is composed of simple right triangles. The particles are like the molecules of the theory; the triangles are its atoms.
Plato's beliefs as regards the universe were that the stars, planets, Sun and Moon move round the Earth in crystalline spheres. The sphere of the Moon was closest to the Earth, then the sphere of the Sun, then Mercury, Venus, Mars, Jupiter, Saturn and furthest away was the sphere of the stars. He believed that the Moon shines by reflected sunlight.