In criminal defense, you can use multiple strategies to try to show innocence, even if those arguments are not mutually exclusive:
For example, if you were trying to defend yourself against a murder charge, I believe you can argue the following:
1. I wasn't even there
2. Even if I was there, I wasn't the actual person to killed him
3. Even if I did kill him, it was an accident
If any one of those holds, you're a free man, but logically you can't argue all three (from a non-legal perspective). In the court room, however, you're free to make these arguments (of course you have to weigh this against the intelligence of the jury, but we've all heard who comprises a jury).
I keep hearing things that seem to exclude this option from a civil suit. Why is that? What can't you argue:
1. We are not infringing (~P)
2. Even if we were infringing, there is prior art, so your patent is invalid (Q)
And in this case, ~P ^ Q is not even a logical contradiction.
You've got to love the law.