This question might be rather academic. It is about patent litigation which is related to the US patent system.
If a patent owner thinks, his patent is being infringed, the patent owner may choose to sue the infringing party to stop his or her activities, as well as to receive compensation for the unauthorized use.
To initiate litigation, patent owner must file a complaint, listing the alleged infringer, the patent(s) allegedly infringed, the infringing products and request relief. This is as simple as filling out Form 18 provided by the US courts and filing it with a district court.
The defendant responds to the plaintiff's complaint by preparing an answer denying the allegations asserted against it, stating defenses to each claim asserted against it and usually asserting counterclaims.
Common defenses are:
- the complaint does not state a claim upon which relief can be granted.
- the products do not infringe the patents-in-suit (35 USC §282(b))
- the patents-in-suit are invalid
- ...and others...
According to FRCP rule 8, there are "defenses" and "affirmative defenses".
Affirmative defenses in nature admit the claims asserted by plaintiff but provide other facts which defeat or mitigate the legal consequences.
There are many answers to complaints where all defenses are listed in a section titled "affirmative defenses", also the non-infrigement defense e.g.:
But how can asserting "non-infringement" be an affirmative defense when the cause of action is infringement? "Non-infringement" is a defense, but a "non-affirmative" defense in my opinion.