Observation is all!
This credo led to a revolutionary insight: everything in the vast space of the universe consists of atoms, i.e. indivisible invisible particles, or aggregations thereof.
Epicurus communicated the atomism conjecture
in a letter to a friend more than two millennia ago. Einstein proved it in 1905.
Although Epicurus' far-sighted views have since ever been challenged, first and foremost by the Church, most of them were ultimately confirmed by science.
Oddly enough, however, the full original document on atomism - including its comforting implications for happiness, death and the soul - is generally little known, even among scientists. It is reproduced here with a topical commentary.
Detlev Schild
Atomismus Geschichte der Physik Geschichte der Chemie