On The Holy Trinity by St. Augustine
Chapter 3.--The Higher Reason Which Belongs to Contemplation, and the Lower Which Belongs to Action, are in One Mind.
3. But that of our own which thus has to do with the handling of corporeal and temporal things, is indeed rational, in that it is not common to us with the beasts; but it is drawn, as it were, out of that rational substance of our mind, by which we depend upon and cleave to the intelligible and unchangeable truth, and which is deputed to handle and direct the inferior things. For as among all the beasts there was not found for the man a help like unto him, unless one were taken from himself, and formed to be his consort: so for that mind, by which we consult the supernal and inward truth, there is no like help for such employment as man's nature requires among things corporeal out of those parts of the soul which we have in common with the beasts. And so a certain part of our reason, not separated so as to sever unity, but, as it were, diverted so as to be a help to fellowship, is parted off for the performing of its proper work. And as the twain is one flesh in the case of male and female, so in the mind one nature embraces our intellect and action, or our counsel and performance, or our reason and rational appetite, or whatever other more significant terms there may be by which to express them; so that, as it was said of the former, |And they two shall be in one flesh,| it may be said of these, they two are in one mind.