Logos
A central Greek philosophical term meaning word, reason, or rational principle — used by Heraclitus, the Stoics, and the early Christians for the order underlying the cosmos.
Logos is one of the most semantically rich terms in Greek philosophy. The word covers a range of meanings — speech, account, reason, ratio, principle — and its use in different traditions tracks different philosophical commitments. Heraclitus speaks of the logos by which all things come to pass, an underlying principle of order intelligible to disciplined thought though hidden from ordinary perception. Aristotle uses logos for the structured rational discourse that philosophy aims at, in contrast to mythos.
The Stoics gave logos its most systematic philosophical role: the cosmos is itself a rational whole, governed by an immanent divine logos, and human reason is a fragment of this cosmic principle. The Gospel of John opens by identifying Christ with the logos (In the beginning was the Word), drawing the philosophical term into Christian theology, where it was further developed by Philo, Justin Martyr, and the Greek patristic tradition.