In the current issue of Lutheran Quarterly (http://www.lutheranquarterly.com/current-issue.html), there are number of very good articles. One of the more interesting ones is a translation by my old classmate Scott Celsor of a piece written by the elderly Gerhard Ebeling on Luther's view of reality. Initially, I read the article with a great deal of skepticism. Ebeling (along with Wilfred Joest) is in some ways the nadir of existentializing Luther interpretation of the last century. His talk of "relational ontology" is not entirely wrong when applied to Luther's statement about righteousness or justification (there is much textual justification for his claims, no pun intended!). Nevertheless, as Oswald Bayer has pointed out, the mistake of much of Ebeling writings are to put all of eggs in the basket of relationalism. Bayer's piece in The Devil's Whore book, I thought was much more accurate. Bayer claims that Luther's ontology is "regional", namely, he utilizes different ontological schemes for the purposes of giving a coherent explanation of what he finds in the Bible. And for the most part though his default mode is an Ockhamist interpretation of Aristotle, which is basically what Graham White has found in his treatment of Luther's late Christological and Trinitarian disputation in Luther as Nominalist. But I digress.
Anyways, what I found insightful is on pages 67-69 where Ebeling talks about the issue of exchange and union within justification and the communicatio idiomatum. And to simplify things a bit, the point that Ebeling makes here is that when one operates with Aristotelian concept of substance as incommunicable form and matter, it really messes up your understanding of Christology and justification. In a word, there can be no real communicatio idiomatum (that is, only a rhetorical or notional one- not a real one) and no concept of the "happy exchange" or iustitia aliena. In what follows, I'd like to draw out the implication for this from my own thinking through the issue yesterday morning.
In Aristotle's philosophy (adopted in one version or another by western Latin theologians after the 12th or 13th century), all individually existing entities in the world (substances) are made up of form and matter. Form is the inner reality of a thing that makes it what it is. Forms are united with matter and give shape to that matter, moving it from a state of potentially being a particular reality (potency) to fully being that reality (act). The classic example of the unity of form and matter is a statue. The image which shapes the statue is its form. The metal or stone that makes up the statue is its matter.
Now for ordinary realities we find in the world, one could of course do much worse for an ontological scheme. When applied to things theological, a number of difficulties arise. The chief problem is that realities in the Aristotelian metaphysic are in a fundamental sense incommunicable. Allow me to explain further.
Since all entities in the world are made up of form and matter, there is an iron law of identity (found particularly in Aristotle's logic). No reality can communicate itself to another reality without becoming part of it in some sense. If I eat a banana, the banana is broken down and is assumed into my matter, and informed by my form. Moreover, a substance can take on new qualities (called "accidents"), but accidents adhere in a substance. For example, one has a bucket of red paint, which is a substance. Now the red paint is painted onto a wall. The redness and the paint are in a sense assumed into the substance of the wall, but they are no longer separate entities. Hence, as Aristotle states in his logic, no two substances can subsist together at the same time. One must replace another or be absorbed into it.
Although of course different realities in the world cannot communicate with one another in the sense of participation, they can nonetheless share qualities. The red paint shares its quality of redness with the wall that has been painted. Similarly, the act of knowing means that a copy of a particular object in the world is imprinted on the intellect. The passive intellect filters out the accidental qualities, while the active intellect identifies the form of the reality. In doing this, the form of the reality is pressed in and imprinted on the mind. The form itself is not in the intellect, but an impression of it is. The form has communicated itself to the mind by making the mind similar to the object observed. The same thing can be seen in the self-communication of form to matter. Matter is imprinted with the reality of the form, without being transmuted into form.
In summary: for Aristotelianism, the possibilities of ontological self-communication are extremely limited. They are limited to the following: 1. One entity composed of form and matter replaces or absorbs another entity composed of form and matter. 2. One entity imprints itself on another entity, and makes the qualities of that entity similar to itself. In both cases, there is no genuine self-communication, or participation of one reality in another.
Now this all seems very abstract, but let's apply these assumptions to the theology and see what we get. In the Middle Ages, the most obvious result of this ontological scheme is the doctrine of Transubstantiation. In other words, if there is a real presence, and two substances can't be present at the same time (bread and wine and body and blood), then logically one must replace the other. The innovation in this regard is quite easily documentable in the Middle Ages. The Patristic understandings of the Lord's Supper worked on the analogy of the Incarnation. This is one of the reasons why many of the Ante-Nicene Fathers adopted the view that consecration was effected by calling on the Holy Spirit (epiclesis). The Holy Spirit (in a sense) "incarnated" the body and blood of Christ in the bread and wine, just like it had worked the original Incarnation! Self-communication was possible for the older tradition (body and blood through bread and wine), whereas the adoption of some version of Aristotelianism made it more difficult to maintain made it more difficult to maintain in the Middle Ages.
When turning to the communicatio idiomatum and justification, the effects become even more obvious. The two natures can be united with one another in a single person, but they cannot communicate with one another in any meaningful sense. If the divine nature communicated its glory to the human nature, then according the scheme of form and matter, the form of the divine nature would inform the form and matter of the human nature. The result would be the absorption of the human nature into the divine (in some sense). Hence, the communicatio idiomatum for Aquinas and most of the rest of the medieval theologians is largely notional or rhetorical. Likewise, to make up for the deficit of the communicated divine glory (found in the formulation of the Greek Church Fathers), the medieval theologians (Aquinas in particular) argued that the human nature was replete with divinely created gifts that imprinted God's moral qualities on the human Jesus (this idea made its reappearance with the Reformed scholastic in the notion of "communicable and incommunicable divine attributes"). Since righteousness and goodness are qualities that inhere in a subject (God) they can only be transmitted through imprinting on, or creating a copy of themselves in the other (Christ's humanity)- not in a real self-communication or participation (i.e., like the genus majestaticum of later Lutheranism!).
As Schleiermacher rightly points out in his schematization of the four natural heresies, what a person says about the relationship between the two natures in Christ will usually (but not always) determine how you understand the saving relationship between God and humanity. And so, if entities are not really communicable to one another except by imprinting a copy or a similar quality to another substance, this utterly destroys any possible of a "happy exchange" between Christ and the believer, or any notion of iustitia aliena. Christ's righteousness could never be transmitted to the believer because righteousness as a quality adheres in him as a subject and cannot be transferred to another subject without becoming a part of that subject. Therefore, the only way that Christ's righteousness could be communicated would be for it to be imprinted or copied into another subject through the giving of a series of capacities or qualities. Hence, as can be observed, the understanding of the possibilities of communication between the divine nature and human natures in the form of created gifts of grace is reproduced in the Thomistic/Roman Catholic understanding of justification. The created gifts of grace or infused capacities imprinted into human nature through baptism are merited by Christ and are given to believers. Such righteousness is not alien righteousness, but represents real qualities and capacities that inhere in the believer as a subject. They make a person like Christ and therefore capable of pleasing God as Christ does.
By contrast, Luther abandons this whole schema for the biblical understanding of God. The God of the Bible is a self-communicating and relational God. God's righteousness is his right relationship with himself and with human beings (the actual meaning of the Hebrew word "Tzedek" is right relationship). Hence, it is intrinsically covenantal and relational. Righteousness is not a quality, but a relationship. God is righteous in that he fulfills his covenant promises both to enforce death on those who violate the law, and to give life and save through the gospel (i.e., the content of the Abrahamic and Sinaitic covenants!). And because righteousness is a relationship, and not a quality, it can be shared. A relationship can be shared, a quality can adhere in two subjects, but it cannot be shared. My wife and I both have brown hair, but it is not the same brown hair! By contrast, we share a common marriage, and live out of that common relationship which we possess with one another. And hence, when the Bible talks about Gods' covenant righteousness, it uses the image of God as the bridegroom and the people of God as his bride.
And so, as Luther puts it in Freedom of a Christian, in God's supreme act of loyalty to his promises, he shares his own righteousness with human beings (i.e., "the righteousness of/from God", Romans 1:17) and in exchange receives their real unrighteousness into himself (i.e., the "happy exchange"). Christ does this by taking on and sharing the wrong relationship that unbelieving and fallen human beings have with the Father, and giving them his own right relationship to him. And justification as the happy exchange is rooted in a real and not merely notional concept of the communicatio idiomatum. The divine person in becoming human incorporates within itself the death and suffering the human nature through the communication of actions. The human nature receives within itself the self-communication of all the divine glory, so that by the divine power present within it, it may by its redeeming and creative actions work salvation. Therefore, just as there can be a real exchange of realities in the Incarnation, there can be a real exchange of sin and righteousness in the happy exchange.