Democritus (ca. 460 BC – ca. 370 BC) was an Ancient Greek philosopher born in Abdera, Thrace, Greece. He was an influential pre-Socratic philosopher and pupil of Leucippus, who formulated an atomic theory for the cosmos. His exact contributions are difficult to disentangle from his mentor Leucippus, as they are often mentioned together in texts.

Their speculation on atoms, taken from Leucippus, bears a passing and partial resemblance to the nineteenth-century understanding of atomic structure that has led some to regard Democritus as more of a scientist than other Greek philosophers; however their ideas rested on very different bases.

Largely ignored in ancient Athens, Democritus was nevertheless well known to his fellow northern-born philosopher Aristotle. Plato is said to have disliked him so much that he wished all his books burned. Many consider Democritus to be the "father of modern science".

Interestingly, Democritus used his theory on atoms to explain the way the universe functions. The philosopher believed that humans only created gods to make sense of incomprehensible things around them. The universe, in his view, is a very comprehensible machine made of atoms. He also maintained that every human’s soul is made of atoms. Democritus believed that human life should be guided by integrity and social responsibility. He maintained that the way to contentment is through discipline, acceptance, moderation, and harmony.

Expanding his theory further, Democritus noted the existence of infinite and eternal atoms and thoroughly postulated about how and why atoms combine to build larger structures in the universe. He gave an intricate explanation on how planets, stars and other worlds formed. Democritus lived in 5th Century BC, but the ideas he created were so advanced that he was often regarded as the founder of modern science.

After returning to his native land he occupied himself with natural philosophy. He traveled throughout Greece to acquire knowledge of its culture. He mentions many Greek philosophers in his writings, and his wealth enabled him to purchase their writings. Leucippus, the founder of the atomism, was the greatest influence upon him. He also praises Anaxagoras. Diogenes Laertius says that he was friends with Hippocrates. He may have been acquainted with Socrates, but Plato does not mention him, and Democritus himself is quoted as saying, "I came to Athens and no one knew me." Aristotle placed him among the pre-Socratic natural philosophers.

As for the moniker, the "Laughing Philosopher," there a few conflicting stories and impressions of Democritus. It is most likely due to his emphasis on cheerfulness although Seneca wrote a darker interpretation saying that he would express his contempt for the follies of those around him by laughing at them. Democritus was also known as "the mocker." This laughter could have come as a result of his undying determinist and materialist composition. 

His ethics were centered on personal integrity and social responsibility with no otherworldly or supernatural influences. (This was another point of contention that Plato may have felt deeply.) His emphasis on cheerfulness was an emphasis on the 'good' or an absence of fear. Instead of doing 'good' because there exists a fear of the law, Democritus felt that people were able to be driven by an interior motive to not shame themselves or others. The goodness in people was not innate but only produced through practice and discipline and that in the presence of the wicked, one has a propensity to do the same. Democritus' cheerfulness also took form in a kind of hedonism albeit moderated and can be seen as being influential to Epicurus.

Modern science has focused on mechanistic questions, which have led to scientific knowledge, especially in physics, while teleological questions can be useful in biology, in adaptationist reasoning at providing proximate explanations, though the deeper evolutionary explanations are often held to be thoroughly mechanistic. The atomists looked exclusively for mechanistic questions, and only admitted mechanistic answers. Their successors until the Renaissance became occupied with the teleological question, which arguably hindered progress.

He died around 370 BC