On The Holy Trinity by St. Augustine
Chapter 1.--Of What Kind are the Outer and the Inner Man.
1. Come now, and let us see where lies, as it were, the boundary line between the outer and inner man. For whatever we have in the mind common with the beasts, thus much is rightly said to belong to the outer man. For the outer man is not to be considered to be the body only, but with the addition also of a certain peculiar life of the body, whence the structure of the body derives its vigor, and all the senses with which he is equipped for the perception of outward things; and when the images of these outward things already perceived, that have been fixed in the memory, are seen again by recollection, it is still a matter pertaining to the outer man. And in all these things we do not differ from the beasts, except that in shape of body we are not prone, but upright. And we are admonished through this, by Him who made us, not to be like the beasts in that which is our better part -- that is, the mind -- while we differ from them by the uprightness of the body. Not that we are to throw our mind into those bodily things which are exalted; for to seek rest for the will, even in such things, is to prostrate the mind. But as the body is naturally raised upright to those bodily things which are most elevated, that is, to things celestial; so the mind, which is a spiritual substance, must be raised upright to those things which are most elevated in spiritual things, not by the elation of pride, but by the dutifulness of righteousness.