Heraclitus was known as the "weeping philosopher", due to his apparent melancholy, which in part caused him to never be able to finish writing out his full thought. He believed the world was in constant change, and the way to understand the world was by listening to the "Logos", something like Reason, or the natural ordering of the world. This was done by investigating the world to see what it revealed to us. He believed that the world was in a state of strife, and understanding the tension between forces was paramount, as they would give way to the Logos. Fire, for Heraclitus, was the force of tension that kept things together, and transitioned from thing to thing. Hegel believed that his conception of opposing forces coming together was the first time someone understood something close to the Hegelian Dialectic. In many of the fragments that have come down to us, he criticized the people for not listening to the Logos, and seems to have a general disdain for most people. Apparently he spent much of the last years of his life living alone away from the city.
Democritus was known as "the laughing philosopher", due to his cheerful disposition. He was one of the first to advance an atomistic metaphysics, believing that the world was composed solely of small atoms, which could not be divided any further.