I've been reading Summa Theologica, First Part of the Second Part, Question 81, Article 1: "Whether the first sin of our first parent is contracted by his descendants, by way of origin?" (http://www.newadvent.org/summa/2081.htm)

In an effort to test my understanding, I'm trying to rephrase Aquinas's position in terms of von Neumann machines.  This is probably a little kooky.  I'm wondering if it is at all on the right track.  (I have no emotional attachment to this, so feel free to tear it to shreds.)

The picture that I'm getting is something like the following:  Human Nature is like a computer program.  Adam was like a von Neumann machine that ran the first instance of this program, and who built subsequent instances.  A soul is a particular instantiation of this program in an individual machine.  Original sin came into the program itself, and hence into Adam, and hence into each of us via Adam because he built the machines that built the machines that ... that built the machines that built us.

(I'm calling us "machines" here just because I'm thinking of us as like von Neumann machines in some sense for purposes of illustration.  I realize that we are not literally machines under A-T.)

The point is, it was the program, the nature common to us all, that sinned, and not just the particular instance of the program in Adam.  That is how each of us can have guilt.

This raises the question of when the damage to the nature occurred.  At first, it seems that, if Adam's nature had changed from undamaged to damaged after he was alive, then he would have changed his substantial form and so become a different kind of thing.  That is, he would have been destroyed at the time of his sin and replaced by something else of a different kind.  But this doesn't seem like the intended story.

This suggests that there is an "inner program" and an "outer program".  The inner program is the substantial form itself, which cannot change, any more than the form of "triangle" can change.  Nothing can change this substantial form.  Things can only go from having it to not having it, or vice versa, and nothing can do even that while remaining the same thing.

Then there is the "outer program", which includes something like "accidents" on the substantial form itself.  This outer part can change without changing the underlying substantial form.  Moreover, the actions of a particular instance of the program can change the outer part of the program, and the instance can do this while remaining the same thing.  It is the program as a whole, including the outer part, that belongs to "human nature", and which gets transmitted by each generation as it builds the next.

So the story now is something like this:  The entire program is instantiated in Adam.  Adam, running this entire program decides to sin in a particular way that re-writes part of the program, damaging it.  That is, the program has decided to damage itself via the instance of itself called Adam.  However, this damage has not changed the "inner part" of itself, the part that determines a thing running the program to be a human.  Nonetheless, the damage has rewritten the program in a way that gets propagated to all the future instances that descend from the first instance.

After reading the other articles in ST under this question, I think that the story above holds up.  Under Thomism, there are something like "accidents" in human nature.  The Thomist jargon seems to be in this quote in Article 2:

"Now just as something may belong to the person as such, and also something through the gift of grace, so may something belong to the nature as such, viz. whatever is caused by the principles of nature, and something too through the gift of grace. In this way original justice, as stated in the I, 100, 1, was a gift of grace, conferred by God on all human nature in our first parent. This gift the first man lost by his first sin. Wherefore as that original justice together with the nature was to have been transmitted to his posterity, so also was its disorder."

So, my "inner part" is what "belong[s] to the nature as such, viz. whatever is caused by the principles of nature", while my "outer part" is what "[belongs to the nature] through the gift of grace".

Does this seem to be in the right ballpark?