The ruling principle. Marcus Aurelius 6.8

Marcus describes the Stoic logos that rules our conscious mind, in the form of human reason, and the entire universe, as reason writ larger than any particular part of the Whole.

Τὸ ἡγεμονικόν ἐστι τὸ ἑαυτὸ ἐγεῖρον καὶ τρέπον καὶ ποιοῦν μὲν ἑαυτὸ οἷον ἂν καὶ θέλῃ, ποιοῦν δὲ ἑαυτῷ φαίνεσθαι πᾶν τὸ συμβαῖνον οἷον αὐτὸ θέλει.

The ruling principle of a thing is that which rouses and steers it, giving it the form that it desires and making every event legible to it in signals that it is willing to read.