After returning to his native land he occupied himself with natural philosophy. He travelled throughout Greece to acquire a 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. The tradition that he was friends with Hippocrates seems to have been based on spurious letters. 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.. Though Aristotle viewed him as a pre-Socratic., it should be noticed that since Socrates was born in ca. 469 BC (about 9 years before Democritus), it is very possible that Aristotle's remark was not meant to be a chronological one, but directed towards his philosophical similarity with other pre-Socratic thinkers.
The many anecdotes about Democritus, especially in Diogenes Laërtius, attest to his disinterestedness, modesty, and simplicity, and show that he lived exclusively for his studies. One story has him deliberately blinding himself in order to be less disturbed in his pursuits; it may well be true that he lost his sight in old age. He was cheerful, and was always ready to see the comical side of life, which later writers took to mean that he always laughed at the foolishness of people.
He was highly esteemed by his fellow-citizens, "because," as Diogenes Laërtius says, "he had foretold them some things which events proved to be true," which may refer to his knowledge of natural phaenomena. According to Diodorus Siculus, Democritus died at the age of 90, which would put his death around 370 BC, but other writers have him living to 104, or even 109.
By convention there is sweet, by convention there is bitterness, by convention hot and cold, by convention color; but in reality there are only atoms and the void.
Aristotle tells us that this theory of matter, commonly called atomism, was a reaction to Parmenides, who denied the existence of motion, change, or the void. Parmenides argued that the existence of a thing implied that it could not have "come into being", because "nothing comes from nothing". Moreover, he argued, movement was impossible, because one must move into "the void" and (as he identified "the void" with "nothing") the void does not exist and cannot be "moved into". His main contribution to chemistry was the suggestion of the atom which he called "atomos".
Democritus explained senses along these lines as well. He hypothesized that different tastes were a result of differently shaped atoms in contact with the tongue. Smells and sounds could be explained similarly. Vision works by the eye receiving "images" or "effluences" of bodies that are emanated. He stated that, "Sweet exists by convention, bitter by convention, color by convention; but in reality atoms and the void alone exist." This means that senses could not provide a direct or certain knowledge of the world. In his words, "It's necessary to realize that by this principle man is cut off from the real." Later philosophers use this to deny that any reliable knowledge can be obtained, but Democritus felt differently:
One view purports that this finer form is reasoning, although Democritus does not explain reason's place in the atomistic view.
He was also a pioneer of mathematics and geometry in particular. We only know this through citations of his works (titled On Numbers, On Geometrics, On Tangencies, On Mapping, and On Irrationals) in other writings, since most of Democritus' body of work did not survive the Middle Ages. Democritus was among the first to observe that a cone or pyramid has one-third the volume of a cylinder or prism respectively with the same base and height.
"In some worlds there is no Sun and Moon while in others they are larger than in our world and in others more numerous. In some parts there are more worlds, in others fewer (...); in some parts they are arising, in others failing. There are some worlds devoid of living creatures or plants or any moisture."
There are two kinds of knowing, the one he calls “legitimate” (gnesie: genuine) and the other “bastard” (skotie: obscure). The “bastard” knowledge is concerned with the perception through the senses, therefore it is insufficient and subjective. The reason is that the sense-perception is due to the effluences of the atoms (aporroai) from the objects to the senses. When these different shapes of atoms come to us, stimulate our senses according to their shape, and there from arise our sense-impressions. (Fr. 135, Theophrastus De Sensu 49-83).
The second sort of knowledge, the “legitimate” one, can be achieved through the intellect, in other words, all the sense-data from the “bastard” must be elaborated through reasoning. In this way one can get away from the false perception of the “bastard” knowledge and grasp the truth through the inductive reasoning. Therefore, the man after taking into account the sense-impressions, can examine the causes of the appearances, draw conclusions about the laws that govern the appearances, and find out the causality (aetiologia) by which they are related. This is the procedure of thought from the parts to the whole or else from the apparent to non-apparent (inductive reasoning). “ But in the Canons Democritus says there are two kinds of knowing, one through the senses and the other through the intellect. Of these he calls the one through the intellect ‘legitimate’ attesting its trustworthiness for the judgement of truth, and through the senses he names ‘bastard’ denying its inerrancy in the discrimination of what is true. To quote his actual words: Of knowledge there are two forms, one legitimate, one bastard. To the bastard belong all this group: sight, hearing, smell, taste, touch. The other is legitimate and separate from that. Then, preferring the legitimate to the bastard, he continues: When the bastard can no longer see any smaller, or hear, or smell, or taste, or perceive by touch, but finer matters have to be examined, then comes the legitimate, since it has a finer organ of perception.” (Fr. 11 Sextus, Adv. Math. VII, 138).
“ In the Confirmations .. he says: But we in actuality grasp nothing for certain, but what shifts in accordance with the condition of the body and of the things (atoms) which enter it and press upon it.” (Fr. 9 Sextus Adv. Math. VII 136).
“ Democritus used to say that 'he prefers to discover a causality rather than become a king of Persia'.” (Fr.118)
(Excerpt from Democritus' Gnoseology 'Handbook of Greek Philosophy: From Thales to the Stoics Analysis and Fragments', Nikolaos Bakalis, Trafford Publishing 2005, ISBN 1-4120-4843-5.
Although Democritus is best known as the propounder of atomism, most of his extant fragments actually relate to the field of ethics. The excerpts are notably fragments quoted by other authors (mostly Stobaeus) and attributed to Democritus, who is also known as "The Laughing Philosopher" (for laughing at human follies) and by his fellow citizens as "The Mocker". Modern scholars credit Democritus with being "the earliest thinker reported as having explicitly posited a supreme good or goal, which he called 'cheerfulness' or 'wellbeing', and which he appears to have identified with the untroubled enjoyment of life ([DK 68] B 188: 'Joy and sorrow are the distinguishing mark of things beneficial and harmful'...). According to Democritus' philosophy, this supreme good was to be achieved through moderation in the pursuit of pleasure, distinguishing useful pleasures from harmful ones, and conforming to conventional morality. This seems to constitute "a recommendation to a life of moderate, enlightened hedonism" similar to that presented by Socrates in Plato's Protagoras and later made famous by Epicurus.
Among numerous examples of the Democritean ethical fragments are the following: