How do we sense objects?  Our instincts tell us that objects are just there, waiting for us to come along to sense them.

Augustine’s instincts were otherwise.  He admitted that “the sense does not proceed from the body that is seen but from the body of the sentient living being to which the soul is adjusted in its own wonderful fashion,” yet the object or “body” plays a role in the formation of a sensation: “sight is begotten of the body which is seen” or more fully “the sense itself is formed by it so that it is no longer just the sense, which can remain entire even in the dark provided the eyes are unharmed, but it is not the informed sense which we call actual sight.”  Thus, “sight is begotten of the visible thing but not from it alone; only if there is a seeing subject present.”  The body forms the sense by “imprinted” on sense.

When the object is removed, the sensation doesn’t remain, though the sense itself does.  Therefore, Augustine does not believe that objects beget senses themselves.  They “beget a form as a likeness of itself,” and this “occurs in the sense when we sense anything by seeing.”  That is, the presence of the object creates a sense experience within the sense.

The Latin of the key phrases is: tamen ex corpore quod videtur gignitur visio, id est, sensus ipse formatur and Gignitur ergo ex re visibili visio, sed non ex sola, nisi adsit et videns and illa tamen informatio sensus, quae visio dicitur, a solo imprimatur corpore quod videtur, id est, a re aliqua visibili .

Objects are not just passively waiting, but join with sensing subjects in the formation of sense experience.  From that combination, sensation is born.

Augustine later pulls the rug out from under this when he warns against becoming entangled in external trinities of sensation.  While the body seen is parent to the sensation of sight, it is only a “quasi-parent” ( quasi parens est forma corporis ex qua fit. Sed parens illa non vera ).

All this is from de Trinitate , Book 11.