Logos (Greek, "word," "reason," "ratio"), in ancient and especially in medieval philosophy and theology, the divine reason that acts as the ordering principle of the universe.
The 6th-century BC Greek philosopher Heraclitus was the first to use the term Logos in a metaphysical sense. He asserted that the world is governed by a firelike Logos, a divine force that produces the order and pattern discernible in the flux of nature. He believed that this force is similar to human reason and that his own thought partook of the divine Logos.
In Stoicism, as it developed after the 4th century BC, the Logos is conceived as a rational divine power that orders and directs the universe; it is identified with God, nature, and fate. The Logos is "present everywhere" and seems to be understood as both a divine mind and at least a semiphysical force, acting through space and time. Within the cosmic order determined by the Logos are individual centers of potentiality, vitality, and growth. These are "seeds" of the Logos (logoi spermatikoi). Through the faculty of reason, all human beings (but not any other animals) share in the divine reason. Stoic ethics stress the rule "Follow where Reason [Logos] leads"; one must therefore resist the influence of the passionslove, hate, fear, pain, and pleasure.
The 1st-century AD Jewish-Hellenistic philosopher Philo Judaeus employed the term Logos in his effort to synthesize Jewish tradition and Platonism. According to Philo, the Logos is a mediating principle between God and the world and can be understood as God's Word or the Divine Wisdom, which is immanent in the world.
At the beginning of the Gospel of John, Jesus Christ is identified with the Logos made incarnate, the Greek word logos being translated as "word" in the English Bible: "In the beginning was the Word, and the Word was with God, and the Word was God. . . . And the Word became flesh and dwelt among us . . ." (John 1:1-3, 14). John's conception of Christ was probably influenced by Old Testament passages as well as by Greek philosophy, but early Christian theologians developed the conception of Christ as the Logos in explicitly Platonic and Neoplatonic terms (see Neoplatonism). The Logos, for instance, was identified with the will of God, or with the Ideas (or Platonic Forms) that are in the mind of God. Christ's incarnation was accordingly understood as the incarnation of these divine attributes.