On The Holy Trinity by St. Augustine
Chapter 6.--The Opinion Which the Mind Has of Itself is Deceitful.
8. But the mind errs, when it so lovingly and intimately connects itself with these images, as even to consider itself to be something of the same kind. For so it is conformed to them to some extent, not by being this, but by thinking it is so: not that it thinks itself to be an image, but outright that very thing itself of which it entertains the image. For there still lives in it the power of distinguishing the corporeal thing which it leaves without, from the image of that corporeal thing which it contains therefrom within itself: except when these images are so projected as if felt without and not thought within, as in the case of people who are asleep, or mad, or in a trance.