relating to or denoting the speculative philosophers active in the ancient Greek world in the 6th and 5th centuries bc (before the time of Socrates), who attempted to find rational explanations for natural phenomena. They included Parmenides, Anaxagoras, Empedocles, and Heraclitus.
a Presocratic philosopher.
Presocratic in other Oxford dictionaries
Definition of Presocratic in the US English dictionary