Democritus of Abdera (fifth century): Greek philosopher, founder of the atomic theory.
The philosopher Parmenides of Elea had opposed "being" to "not being", and had pointed out that change was impossible, because it would mean that something that was "not being" changed into "being", which he thought was absurd. This was a profound idea and many philosophers tried to solve this paradox.
One solution was the hypothesis of Democritus of Abdera: matter is made up from atoms. There was no real evidence for this idea (which was not completely new), but it explained why change was possible. The atoms were always moving and clustering in various, temporary combinations. Therefore, things seemed to change, but "not being" never changed into "being". (It was assumed that "not being" was a vacuum, which means that it is in fact not a "not being" because a vacuum exists in four dimensions.) The consequence of this idea is that we are allowed to use our senses, although Democritus warns us to be careful.
Later generations did not hesitate to attribute miracles to the sage of Abdera. It was believed that he had once saved the people of Abdera from pestilence. Democritus is also known to have published a book about chameleons.
A biography was included in the Lives of Eminent Philosophers by Diogenes Laertius (here).