Ordinary eyesight involves eyes, light, physical distance, direction and, in many cases, facing the object. The opposing argument treats these observed conditions as necessary for every vision and concludes that divine vision would require God to be in a place or direction.
The reply distinguishes customary conditions from the essence of vision. The fact that an act of perception ordinarily occurs under certain conditions in this world does not establish that no other manner is rationally possible. Without this distinction, divine transcendence and vision appear to conflict unnecessarily.