(Canto 4, lines 136-138)
Democritus (ca. 460-370 BC), Greek thinker who first formulated the "atomistic philosophy", according to which the universe is an atomic structure assembled from random chaos according to mechanical laws.
It is believed that "Diogenes" can refer either to Diogenes, founder of Cynic philosophy (c. 412-323 BC) or his namesake Diogenes of Apollonia, representative of the Ionian school. Commentator Reggio believes the latter to be the more likely candidate, since he is referred to several times by Aristotle, and also mentioned by St. Thomas in connection with Anaxagoras and Thales, like here.
Anaxagoras (c. 500-428 BC) is renowned as the first to bring philosophy to Athens. He is famous for his theory of reality as consisting of an innumerable amount of fragments or "seeds" and their combination and division. He is said to have been banished for denying the divine nature of the sun and moon.
Thales is remembered as the founder of the Ionian school of philosophy and for his belief that water is the principal constituting element of the universe. He is also famous for his study of eclipses.
Commentators are not in agreement whether the Zeno referred to here is the Stoic Zeno (334-262 BC) or Zeno of the Eleatic school (5th century BC). It is possible that Dante, whose knowledge of Greek philosophers derived from the works of Aristotle, might have thought they were one and the same. For example, the Stoic Zeno is mentioned in the Convivio, but characterized there as "the first and most important" of the "very ancient philosophers", which seems incorrect as the Stoic Zeno was a later philosopher than Aristotle.
Empedocles (ca. 490-430 BC) believed that the universe consisted of the four elements fire, air, water and earth, whose combination and division according to the opposing principles of "love" and "strife" was the source of all things and their transformations. He was also a poet, whose works, like those of Homer, were sung at the Olympic games. According to a legend, he committed suicide by throwing himself into the volcano Etna during an eruption.
Heraclitus (ca. 535-475 BC) is remembered for his belief that everything in the universe "flows" or changes, by the force of some divine, eternal power. He and Empedocles are classified as "pre-Socratic" philosophers.