Philosophy is commonly defined as the search for truth about the origins and nature of the universe, the world, and humanity. Although there is some dissention among critics about specifics, it is generally agreed that this search began in the ancient Greek world during the sixth century B.C. Even before this, however, in the Theogony (725 B.C.), in which the poet Hesiod retells the myths of the gods, the origins and order of the universe were subjects of speculation. One characteristic of early Greek philosophy is that it began to focus decreasingly on mythological solutions to the problems at hand; the issues examined by Hesiod were rethought in terms of the natural world as opposed to the mythological. Scholars also note that while these issues were addressed in the Greek world, the development of mathematics, science, and astronomy in neighboring countries, including Babylonia and Egypt, contributed significantly to the development of intellectual and philosophical thought in Greece.
The Greek city of Miletus in the Greek state of Ionia is generally pinpointed as the epicenter of philosophical inquiry. Such critics as W. K. C. Guthrie explain that the environment of Miletus provided both the leisure and the stimuli necessary for the development of intellectual thinking. Guthrie notes that the culture was materialistic and that intellectuals began to recognize that the success, prosperity, and good fortune of the city resulted from the hard work of people rather than favor of the gods.
Within this background and these influences, several individuals began to search for order and unity in the apparent chaos of nature. The first of these was Thales of Miletus. Little is known for certain about his life or beliefs, despite some of Aristotle's interpretations of Thales's thought. In fact, the authority of Aristotle's views with regard to Thales and later Presocratic philosophers is an issue frequently debated among twentieth-century critics. Aristotle argued that Thales believed that the substance of the world from which all things derived was water. Such twentieth-century critics as H. F. Cherniss doubt that this is what Thaïes really meant. Similarly, G. B. Kerferd and others agree that Thales's importance as the founder of philosophy in ancient Greece may have been over-emphasized in the past.
The next Ionian philosopher, Anaximander of Miletus, had a number of interestsaside from philosophy, including geography. In his search for the primary substance of life, Anaximander developed his theory of the apeiron, often translated as "that which is unlimited" or "unbounded." He described the apeiron as ageless, deathless, indestructible, controlling or guiding all things. These characteristics were traditionally ascribed to Greek gods, thus leading later critics to another arena of debate. Although Anaximander was searching for a natural explanation for the origins of the world, his solution bore a number of similarities to the explanations provided by Greek mythology. Kerferd describes two distinct issues addressed by twentieth-century critics, the first being the role of the religious and mythical elements in Presocratic thinking, and the second being the correct assessment of the rational elements in Presocratic philosophy.
Anaximenes of Miletus followed Anaximander in his thinking and is thought to have attempted to explain some of the problematic areas of his predecessor's theory of the apeiron. Anaximenes, however, substituted another substance for Anaximander's apeiron: air. Further, he attempted to account for the differentiation he saw in the world around him through a natural mechanism: the process of condensation and rarefaction.
Heraclitus of Ephesus focused on the problem he found with both Anaximander's and Anaximenes's theories. In the thinking of the Milesians, everything that existed was a different degree, or a rearrangement, of everything else. In Heraclitus's view things were changed—not just rearranged—by a process, and the process itself was everything. Heraclitus was interpreted by Aristotle as believing that the primary substance was fire. Most twentieth-century scholars see things differently, believing that Heraclitus was speaking of fire as a process of change. These later critics also disagree with Aristotle's conclusion that Heraclitus believed that everything is constantly changing. Modern critics tend to agree that Heraclitus meant that the world as a whole is in a state of continuous change and that individual things are subject to change. Additionally, Heraclitus is noted as being the first philosopher to assert that reality is not what the senses perceive. Operating within the process that Heraclitus saw as reality was the soul, which Heraclitus argued was the purest part of the process. He believed it was possible to discover the meaning of the world by examining one's own soul and that it was humankind's duty to perform such an examination.
Pythagoras of Samos was a contemporary of Heraclitus and had a number of views on the soul as well. Pythagoras and his followers believed in the transmigration of the soul—that it was reincarnated repeatedly until it was purified, at which point it entered into a state of divinity. The Pythagoreans also developed cosmological theories, adopted to some degree from the Milesians. The fundamental tenet of their views was that the nature of things was number, the group's study of music is attributed as the cause of this conclusion.
At the same time, Xenophanes of Colophon was developing theological views that brought ridicule from Heraclitus. The two agreed on many points; for example, they both opposed Greek polytheism. But Xenophanes claimed that God is one and that God never moves or changes, whereas Heraclitus believed that everything was part of the process. Critics note that Xenophane's beliefs were developed outside of the mainstream of Greek philosophy.
It is likely that Parmenides of Elea knew of Heraclitus's work, which is believed to have been written in the early part of the fifth century B.C. The two are similar in that they both addressed the Milesian assumption that change and motion occur. Heraclitus attempted to prove that all things are subject to change. Parmenides argued that change is impossible. He focused on logic, being, and the nature of identity, claiming that nothing can come into or out of being. This "Eleatic logic" changed the face of natural philosophy. Parmenides's supporters, including Zeno of Elea and Melissus of Samos, accepted his arguments and conclusions. Melissus developed further arguments refuting the notions of change and plurality, while Zeno supported Parmenides by challenging his critics with paradoxes.
Even Parmenides's detractors used his logic as a starting point. Both Empedocles of Acragas and Anaxagoras of Clazomenae endorsed the doctrine that nothing can come to be from not being. Empedocles argued that fire, water, earth, and air existed. The rearrangement of these bodies accounted for differentiation. Empedocles also maintained that Love and Strife acted on the four physical bodies and thus caused rearrangement among them. Like Empedocles, and following Parmenidian logic, Anaxagoras argued that nothing comes to be but is mingled with and separated from existing things. He went on to expand Empedocles's list of existing things, arguing that every natural substance existed, not just fire, air, water, and earth. These natural substances included hair, stone, flesh, gold, hot, cold, wet, and dry, among other things. Anaxagoras claimed that in everything there was a portion of everything else. He also argued that the origin of motion was not Love and Strife, but nous, or "mind," thus introducing into Greek thought the idea that reason controlled nature.
Melissus attacked both Empedocles's and Anaxagoras's theories and argued that motion of any kind is not possible if there is no void. Leucippus of Miletus then introduced the idea that if no motion is possible without a void, then there must be a void, a "physical nonbeing," as Cherniss describes it. Leucippus and his followers, including Democritus of Abdera, argued against Anaxagoras's nous and Empedocles's Love and Strife as forces that cause motion. Rather, they maintained that constant motion is a characteristic of all matter, thus asserting what their Melesian predecessors had assumed. They went on to argue the other characteristics of matter: its particles are indivisible and unchangeable, and they differ in size and shape. Apparent differentiation was explained as an illusion created by the number, size, shape, arrangement, and position of the atoms. Democritus seemed to recognize a problem with this conclusion, apparently asserting that the mind must be able to discern reality where the senses failed to do so but recognizing that the Atomic system did not account for this distinction. For their theories regarding particles of matter, these thinkers were known as the Atomists.
Protagoras appeared to recognize that Democritus's allusions served as threats to reason. He criticized Eleatic thinking and attempted to show that Eleatic logic could be used to overthrow Eleatic conclusions, as well as the evidence of sense perception. He asserted that what appears to a person to be true is true.
Another attack on Eleatic logic came from Gorgias, who used it to prove that nothing exists, that if something exists it is incomprehensible, and that if it is comprehensible it is incommunicable. Such attacks made Eleatic logic a way for proving and disproving anything, and men who used it in such a way, who studied and refined it as a method of argumentation, became known as Sophists. It is noted that their study contributed to the development of the Socratic dialectic. They were often disdained by other intellectuals for a number of reasons, as Kerferd points out. The Sophists became professionals who sold their teachings. Kerferd argues that the reason they were scorned by others is that they did not discriminate among their students, and some feared what "certain kinds of people" would do with their knowledge about politics and matters of state. Other areas of instruction, in addition to law and politics, may have been geometry, astronomy, physical science, literature, and theology.
There are virtually no extant texts of the Presocratic philosophers. What is known of their work comes to us from a variety of sources, including direct quotations of their work found in the texts of Plato, Aristotle, Simplicius, Plutarch, Sextus Empiricus, Clement of Alexandria, Hippolytus, Diogenes Laertius, and John Stobaeus. Additionally, their work is discussed by Plato, who provided mostly casual and amused comments on their thinking, and by Aristotle, who offered more formal commentary on their work and supplied surveys of Presocratic opinions. Furthermore, in the later part of the fourth century B.C., theophrastus wrote nearly twenty books of Physical Opinions, or philosophical thinking, of the Presocratics. Theophrastus' writings laid the groundwork for other doxographical works, or collections of "opinions," including that of Hermann Diels, who compiled the of-cited and reprinted Die Fragmente der Vorsokratiker in the late nineteenth century.
The impact of each Presocratic philosopher on his predecessors has been a frequent topic of discussion and debate among critics since the time of Plato and Aristotle. As a whole, however, the group of Presocratic thinkers discussed in this entry brought the search for the origins and nature of the universe from its mythological roots to rational conjecture about natural science (sometimes with theological overtones) and finally into the hands of Socrates and his pupils, who focused on the inquiry into the nature of society and its moral and ethical dilemmas.