While we’re at it, stolen shamelessly from my friends at The Sacred Page:
There is in God, as there is in us, a sort of ‘circulation’ (circulatio) in the operations of mind and will: for the will returns to that which understanding initiated. But with us the ‘circle’ (circulus) closes in that which is outside of us: the external good moving our intellect, our intellect moving the will, and the will returning through its appetite and love to the external good. But in God, the ‘circle’ is completed within himself: for when God understands himself, he conceives his Word which is the ‘rationale’ of everything known by him, since he understands all things by understanding himself; and through this Word, he ‘proceeds’ to the love of all things and of himself . . . And the circle being completed, nothing more can be added to it: so that a third procession within the divine nature is impossible, although there follows a procession toward external nature.
–St. Thomas Aquinas, De potentia, q. 9, a. 9.