Because ancient Greek thinkers such as Democritus lacked sophisticated technology and tools such as the microscope, his theory of the atom was due more to thought experimentation than to hard empirical observation, as used in modern science. In essence, he conceptualized it.
The first seeds for Democritus' theory came from his teacher and mentor, Leucippus, seeds that Democritus then adopted and further developed. As with many of the earliest ancient Greek philosophers, particularly the pre-Socratics, Democritus was interested in the discovery of first principles, those substances to which all subsequent substances could be essentially reduced. While earlier thinkers suggested things such as water, air and fire as first substances, Democritus surmised that all matter can be rarefied into small invisible particles called atoms, particles that are solid and indestructible.
According to Democritus, atoms differ in form, size and arrangement, depending on the specific type of substance they produce. Large objects, for example, are made of big rounder atoms, whereas small objects consist of pointy, smaller atoms. For Democritus, reality itself consisted of only two things: the atoms themselves and an enormous void through which the atoms can move and assimilate into different configurations. Consequently, Democritus argued that all of one's sensual experiences — the collection of sense data from the environment — is due to the actual physical contact the human being experiences with atoms inside their unique arrangements. For example, the sense of taste is produced through tiny jagged atoms actually tearing at the surface of the tongue. Additionally, Democritus considered the soul as a collection of loose, smooth atoms that eventually disperse into the atmosphere upon death.