The Greek philosopher Parmenides (active 475 B.C.) asserted that true being and knowledge, discovered by the intellect, must be distinguished from appearance and opinion, based on the senses. He held that there is an eternal One, which is timeless, motionless, and changeless.
Parmenides was born in Elea in southern Italy in the late 6th century B.C. Socrates, in Plato's Thaetetus, tells how as a young man he met Parmenides and Zeno on their visit to Athens about 450. Little else is recorded about the details of Parmenides's life. He wrote a didactic poem in hexameters, the meter of the Homeric epics and of the oracular responses at Delphi, in which he described a divine revelation. Fragments of the poem remain and provide a fair idea of what he attempted to prove, although even when the entire poem was extant there were problems of interpretation.
The poem consists of a prologue and discussions of the Way of Truth and the Way of Opinion. In the allegorical prologue, the narrator is carried on a chariot to the realm of Light by the daughters of the Sun. There he is met by an unidentified goddess whose revelations make up the rest of the work. The Way of Truth is the way of the intellect; it discovers True Being, which is unitary, timeless, motionless, and changeless although spatially limited. Its opposite, Non-Being, cannot be intellectually known and is therefore to be denied as a concept. The contradictory Heraclitean notion of Simultaneous Being and Non-Being is also denied.
The Way of Opinion, which is the usual path of mortals, deals with the evident diversity of nature and the world perceived through the senses. The validity of sense data and of the objects perceived through the senses is denied. Parmenides insists on not confusing the physical objects with those of the intellect, although in the light of this disclaimer his elaborate explanations of various physical phenomena are somewhat puzzling. These explanations, whether they represent a summary of popular beliefs, Pythagorean thought, or Parmenides's own attempts to explain the world in the most plausible way through the use of the (necessarily false) senses, contain a few shrewd observations in an astronomical scheme that is impossible to reconstruct. Underlying all physical reality are the external opposites, Fire and Darkness. A mixture of the two governs the makeup of all organic life.
Parmenides's importance lies in his insistence on the separation of the intellect and the senses. His allegorical discussion of the paths of thought represents the earliest attempt to deal with the problems of philosophical method.
The extant fragments of Parmenides's poem are collected in Hermann Diels, ed., Die Fragmente der Vorsokratiker (1957), translated by Kathleen Freeman in Ancilla to the Pre-Socratic Philosophers (1948) and discussed by her in The Pre-Socratic Philosophers (1946; 3d ed. 1953). Excellent discussions and commentaries on Parmenides are in G. S. Kirk and J. E. Raven, The Presocratic Philosophers (1962), and W. K. C. Guthrie, A History of Greek Philosophy (3 vols., 1962-1969). General discussions of Pre-Socratic philosophy as part of the development of Greek thought may be found in the standard histories of Greek literature, of which a noteworthy example is Albin Lesky, A History of Greek Literature (trans. 1966).