Editing Leucippus (section)

== Philosophy ==
=== Atoms ===
Leucippus is credited with developing the philosophical school of [[atomism]]. He proposed that all things are made up of microscopic, indivisible particles that interact and combine to produce all the things of the world.{{Sfn|Skordoulis|Koutalis|2013|p=467}}{{Sfn|Taylor|1999|pp=181–182}} The atoms postulated by Leucippus come in infinitely many shapes and sizes, although the size and shape of each atom is fixed and unchanging. They are in a state of constant motion and continuously change arrangements with one another.{{Sfn|Zilioli|2020|p=4}}{{Sfn|Barnes|2012|pp=365–366, 440}} He reasoned that there must be infinite types of atoms because there is no reason why there should not be.{{Sfn|Gregory|2020|pp=24, 452}}{{Sfn|Barnes|2012|pp=363–365}}

According to the 4th-century BCE philosopher [[Aristotle]], Leucippus argued that logically there must be indivisible points in everything. His reasoning was that if an object was made entirely of divisible points, then it would not have any structure and it would be intangible.{{Sfn|Hasper|2014|p=66}}{{Sfn|Barnes|2012|p=360}} Leucippus developed atomism along with his student, [[Democritus]];{{Sfn|Skordoulis|Koutalis|2013|p=467}}{{Sfn|Taylor|1999|pp=181–182}} while Leucippus is credited with the philosophy's creation, Democritus is understood to have elaborated upon it and applied it to natural phenomena.{{Sfn|McKirahan|2011|p=304}}

Two works are attributed to Leucippus: ''The Great World System'' and ''On Mind''.{{Sfn|Skordoulis|Koutalis|2013|p=467}}{{Sfn|Graham|2008|p=334}} The former may have originally been titled ''The World System'' and then later renamed to avoid confusion with Democritus's ''The Little World System''. Leucippus's ''The Great World System'' has sometimes been attributed to Democritus.{{Sfn|Cerri|2016|pp=15–16}} Only one extant [[Literary fragment|fragment]] is attributed to Leucippus, taken from ''On Mind'': "Nothing happens at random, but everything for a reason and by necessity".{{Sfn|Gregory|2020|p=34}}{{Sfn|Vamvacas|2009|p=218}} Leucippus believed that all things must happen [[Determinism|deterministically]], as the positions and motions of the atoms guarantee that they will collide in a certain way,{{Sfn|McKirahan|2011|p=320}}{{Sfn|Kirk|Raven|1957|p=418}} invoking the [[principle of causality]].{{Sfn|Barnes|2012|p=413}} This was reminiscent of the 6th-century BCE philosopher [[Anaximander]]'s argument that movement is created by differences, and it was later codified by the 17th-century philosopher [[Gottfried Wilhelm Leibniz]] with the [[principle of sufficient reason]].{{Sfn|Vamvacas|2009|p=40}} Leucippus rejected the idea that there was an intelligent force governing the universe.{{Sfn|Barnes|2012|p=414}}

=== Eleatics and the void ===
Leucippus's atomism was a direct response to [[Eleatic philosophy]]. The Eleatics believed that nothingness, or [[The Void (philosophy)|the void]], cannot exist in its own right. They concluded that if there is no void, then there is no motion and all things [[Monism|must be one]].{{Sfn|Skordoulis|Koutalis|2013|pp=467–468}}{{Sfn|Gregory|2020|pp=23–24}} Leucippus agreed with their logic, but he said that the void did exist, and he was therefore able to accept the existence of motion and [[Pluralism (philosophy)|plurality]].{{Sfn|Vamvacas|2009|p=212}}{{Sfn|Stokes|1971|p=219}} Like the Eleatics, Leucippus believed that everything exists in an eternal state and nothing can come into or out of existence, applying this to both atoms and the void.{{Sfn|Vamvacas|2009|pp=212–213}}{{Sfn|Kirk|Raven|1957|p=405}} Aristotle described Leucippus as saying that atoms are not an addition to the void, but that atoms and the void are two opposites that exist beside one another.{{Sfn|Vamvacas|2009|pp=212–213}} The 6th-century CE philosopher [[Simplicius of Cilicia]] also wrote about this idea, but he attributed it to Democritus.{{Sfn|Graham|2008|p=345}} According to the Christian author [[Lactantius]], Leucippus compared atoms to the particles of floating dust that are visible in sunlight.{{Sfn|Barnes|2012|p=366}}

Leucippus's atomism kept the [[Ontology|concepts of reality]] developed by the Eleatics, but it applied them to a physical explanation of the world.{{Sfn|Vamvacas|2009|p=212}}{{Sfn|Laks|2018|p=89}} By moving away from the abstract points and units of geometry, he formed a possible solution to the [[Zeno's paradoxes|paradoxes of motion]] created by [[Zeno of Elea]], which held that indivisibility made motion impossible.{{Sfn|Furley|1987|p=110}}{{Sfn|Kirk|Raven|1957|p=372}} Leucippus also contested the Eleatic argument against divisibility: that any divider between two objects can also be divided. He argued that the void is a divider that does not have being and therefore cannot be divided.{{Sfn|Stokes|1971|pp=221–222}} Though Leucippus described atoms as being able to touch one another, Aristotle understood this to mean atoms being near one another, as Leucippus maintained that the void must exist between all atoms.{{Sfn|Stokes|1971|p=219}}{{Sfn|Barnes|2012|p=349}}

=== Soul and perception ===
Leucippus and Democritus proposed that heat, fire, and the [[soul]] are made of spherical atoms, as this shape would let them move past one another and cause the others to move more efficiently. They believed in a physical soul that drives motion in living things, and they described respiration as the process of expelling soul atoms and absorbing new ones.{{Sfn|Augustin|Pellò|2021|pp=615–616}} Death then coincides with the last breath, as soul atoms are no longer being replenished. Sleep is a similar state in which a reduced number of soul atoms are in the body.{{Sfn|Augustin|Pellò|2021|pp=617–618}}

Leucippus was the first philosopher to describe a [[Philosophy of mind|theory of thought]] and [[Philosophy of perception|perception]].{{Sfn|McKirahan|2011|p=332}} He described sensory input as a transfer between atoms, created when external atoms come into contact with the atoms of the soul.{{Sfn|McKirahan|2011|p=329}} Leucippus said that sight is caused by a film of atoms emitted from an object, maintaining the shapes of its atoms and creating a reflection of the object in the viewer's eye. His description of vision was inspired by [[Empedocles]], who formed a similar concept of objects emitting films of themselves.{{Sfn|McKirahan|2011|p=330}} Leucippus posited that concepts such as color and texture are created by different arrangements of atoms, and that abstract concepts such as [[justice]] and [[wisdom]] are produced through the arrangement of soul atoms.{{Sfn|Furley|1987|p=171}}

According to Epiphanius, Leucippus said that reasoned [[knowledge]] is impossible to obtain and only unreasoned [[belief]] exists.{{Sfn|Barnes|2012|p=562}} 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".{{Sfn|Vamvacas|2009|p=211}} The 20th-century scholar C. C. W. Taylor said that "we have no evidence to suggest that Leucippus was concerned with [[epistemological]] questions".{{Sfn|Taylor|1999|p=189}}

=== Cosmology ===
Leucippus said that the void extends infinitely, expanding across the entire universe. He also said that there is an infinite number of atoms, spread across the void. The Earth and the [[cosmos]]—including the Sun, the Moon, the stars, and anything else visible in the night sky—exist together in the void.{{Sfn|Furley|1987|p=136}}

Leucippus said that the cosmos was created when a large group of atoms came together and swirled as a vortex. They shifted around each other until they were sorted "like to like". The larger atoms gathered in the center while the smaller ones were pushed to the edge. The smaller atoms became the celestial bodies of the cosmos. The larger atoms in the center came together as a membrane from which the Earth was formed.{{Sfn|Furley|1987|pp=140–141}}{{Sfn|Kirk|Raven|1957|pp=411–412}} Ancient writers disagreed about what Leucippus meant when he described the membrane: [[Aetius (philosopher)|Aetius]] said that the smaller atoms were part of the membrane, encasing the larger atoms, but [[Diogenes Laertius]] said that the larger atoms formed a membrane themselves and the smaller ones were excluded.{{Sfn|Kirk|Raven|1957|pp=411–412}} Leucippus also believed that there were distant cosmoses in other parts of the void; this makes him the first known philosopher to propose the existence of other worlds besides Earth, though some ancient doxographers have attributed these ideas to the earlier [[Ionian philosophers]].{{Sfn|Kirk|Raven|1957|p=412}}

Like other pre-Socratic philosophers, Leucippus believed that the Earth was in the center of the cosmos.{{Sfn|McKirahan|2011|p=327}} He said that the other celestial bodies orbited around the Earth, with the Moon being the closest to the Earth and the Sun being the farthest.{{Sfn|Graham|2008|p=338}} He described the stars as orbiting the fastest.{{Sfn|Graham|2008|p=339}} While initially "moist and muddy", the stars dried and then ignited.{{Sfn|Furley|1987|pp=140–141, 145}}

Leucippus adopted the idea of the Ionian philosophers that the Earth is flat.{{Sfn|Kirk|Raven|1957|p=412}} According to Aetius, Leucippus thought of the Earth as "drum-shaped", with a flat surface and some degree of depth.{{Sfn|Graham|2008|p=339}} 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.{{Sfn|Graham|2008|pp=339–340}} Aetius also tells of Leucippus's explanation for thunder: that it is caused by fire being compressed in clouds and then bursting out.{{Sfn|Graham|2008|pp=340–341}}

Many early philosophers were confused by the fact that earthly objects fell downward while celestial objects moved in a curved trajectory. This prompted many of them to believe in a non-earthly substance that composes the celestial bodies. With his model of the cosmos, Leucippus was able to justify why these entities move differently even though they are made of the same substance.{{Sfn|Furley|1987|pp=146–147}} Leucippus gave no explanation for how motion began, for which he was criticized by Aristotle.{{Sfn|Furley|1987|p=149}}{{Sfn|Kirk|Raven|1957|p=417}} It is unclear whether Leucippus considered vorticies to arise by chance or as a deterministic outcome.{{Sfn|Gregory|2013|pp=459–460}}