Leucippus described the beginning of the cosmos as a vortex of atoms that formed the Earth, the Sun, the stars, and other celestial bodies.

As Leucippus considered both atoms and the void to be infinite, he presumed that other worlds must exist as cosmoses are formed elsewhere.

Leucippus and Democritus described the soul as an arrangement of spherical atoms, which are cycled through the body through respiration and create thought and sensory input.

Most scholars agree that Leucippus existed, but some have questioned this, instead attributing his ideas purely to Democritus.

Two works are attributed to Leucippus (The Great World System and On Mind), but all of his writing has been lost with the exception of one sentence.

[6][8] Some 20th-century classicists such as Walther Kranz and John Burnet have suggested that he lived in all three cities—that he was born in Miletus before studying under Zeno in Elea and then settling in Abdera.

[20] Only one extant fragment is attributed to Leucippus, taken from On Mind: "Nothing happens at random, but everything for a reason and by necessity".

[21][22] Leucippus believed that all things must happen deterministically, as the positions and motions of the atoms guarantee that they will collide in a certain way,[23][24] invoking the principle of causality.

[34] According to the Christian author Lactantius, Leucippus compared atoms to the particles of floating dust that are visible in sunlight.

[35] Leucippus's atomism kept the concepts of reality developed by the Eleatics, but it applied them to a physical explanation of the world.

His description of vision was inspired by Empedocles, who formed a similar concept of objects emitting films of themselves.

[47] The 20th-century writer Constantine Vamvacas said that Leucippus rejected this belief, and that it was the Eleatic philosopher Parmenides who held it.

According to Vamvacas, Leucippus and Democritus "believe that sense experience, however limited, constitutes objective knowledge of the physical world".

[56] He said that the flat Earth is tilted on its horizontal axis so that the south is lower than the north, explaining that the northern region is colder than the southern region, and the cold compacted air of the north can better support the Earth's weight than the warm rarefied air of the south.

[58] Aetius also tells of Leucippus's explanation for thunder: that it is caused by fire being compressed in clouds and then bursting out.

[63] Modern understanding of Leucippus's role in the development of atomism comes from the writings of the ancient Greek philosophers Aristotle and Theophrastus.

[64] Aristotle's 4th-century BCE record of Leucippus and Democritus's philosophy is the oldest surviving source on the subject,[65] though he did not distinguish who developed which atomist ideas.

[72] Ancient atomism was revived in the 16th and 17th centuries,[73][74] especially by proponents of the mechanical philosophy such as Pierre Gassendi (1592–1655) and Robert Boyle (1627–1691).

[75] Nevertheless, in practice experimental chemists such as Boyle rather relied on the tradition of corpuscularianism which had developed in medieval alchemy and ultimately goes back to works such as Aristotle's Meteorology IV.

[79] The 20th-century physicist Werner Heisenberg argued that Plato's theory of forms was closer to the 20th-century understanding of physics than Leucippus's conception of atoms, saying that modern atoms are more like the intangible Platonic forms than the discrete material units of Leucippus.

[10][19] Most modern philosophers agree that Leucippus existed, but there is disagreement on whether his work can be meaningfully distinguished from that of Democritus.

[66] In 2008, the philosopher Daniel Graham wrote that no significant work on the historicity of Leucippus has been produced since the early 20th century, arguing that "recent scholarship tends to avoid the question as much as possible".

[64] Scholars who maintain that Leucippus existed argue that he only taught orally or that any written works he produced were never meant for publication.

[83] Supporting this argument is that Epicurus considered ethics to be foundational to philosophy, and Leucippus had no teachings on that subject.

[5] The existence of Leucippus was an issue in 19th-century German philosophy, where it spawned a debate between Rohde, Natorp, and Hermann Alexander Diels.