I was thinking a little about causation today, and several times I thought I had an idea for a blog post only to see it collapse when I tried to think through things more clearly. So instead I have a broad question.

Imagine the following relatively plausible example. (Only relatively plausible because the computers at Cornell are kept pretty clean.) There’s a virus going around Cornell, call it YourDoom. Both Tamar’s computer and Delia’s computer are infected with it. Both of them send the virus to my computer during the morning which in turn becomes infected. What infected computers do is shout *TROGDOR* at midday. Question: What factors should determine whether Tamar’s computer or Delia’s computer or both caused my computer to shout *TROGDOR* at midday?

Here are some sample issues to consider:

* Which of them sent it first
* Whether the second virus overwrites the first virus on the hard drive, or does nothing to the hard drive if there is a virus there, or writes a second copy of the virus down
* How the virus gets from hard drive to memory

And I imagine there are others. If you think of this case at a fairly abstract level it looks like a case of trumping preemption, but I’m not sure it still looks that way when you get to the details.