This question, if you return to Part 1, is a question about the "weak imaging" of human nature. The question here isn't how we participate in the divine life ("strong imaging"), but how a created good reflects God in its natural glory. Stars have their glory. So do trees and butterflies. So what reflects God's imprint in human nature given its capacity to change and grow?
Tanner answers the question by pointing to God's incomprehensibility:
One can say human nature in this respect forms an image of that divine image by imitating God's own incomprehensibility...God is incomprehensible, beyond human powers of positive explication through concepts and speech, because God is without limits or bounds...Humans imitate God's incomprehensibility by having a nature that is also in a sense unlimited, unbounded by a clearly delimited nature, in virtue, in the human case, of an expansive openness and initial indefiniteness apart from some more specific formation from without that our own self-reflective capabilities help to direct.
Tanner pushes this even further, pointing toward the "mystery" of human nature, how we still wrestle with great unknowns about who and what we are. Nature versus nurture. The mind/body problem. The origins and nature of consciousness. And so on. These mysteries of human nature point toward the image of God. Tanner highlights how these apophatic--unknown, unsayable--aspects of human nature point toward the mystical, ineffable, mysterious, apophatic aspects of God:
Whatever the knowable dimensions of human nature, its apophatic ones are what count here for imaging of God. An apophatically-focused anthropology form the natural consequences of an apopothatic theology.
Which brings us back to the title of Tanner's book, how Christ is our key here. The incarnation is an apophatic mystery. We cannot specify, in human terms, how Jesus was fully both God and man. That union is a mystery. And that mystery is what sits at the heart of human nature when it is strongly imaging God. Psychology and neuroscience cannot describe how humans reflect, share, and participate in the divine life. This mystery is beyond scientific description. And in this way, the mystery of our union with God, the mystery of human nature, becomes an incarnational sign within creation pointing toward the second person of the Trinity:
Jesus is not the comprehensible stand-in or substitute for an incomprehensible divinity but the very exhibition of the incomprehensible divinity of the Word in human form or medium. Jesus displays in his life what it means to be an incomprehensible image in the flesh of an incomprehensible God.
Following the paradigm of Christ, then, there would be something incomprehensible about human nature as it is shaped by a relationship with God, too, which makes it the imitation of God. We are an incomprehensible image of the incomprehensible both in those natural capacities that allow us to be radically re-formed, and in what we become in relation to the true image, the Word incarnate.
