Affidavits of Want of Knowledge
When answering a plaintiff's complaint in litigation, the Illinois Code of Civil Procedure gives a defendant three options. He can either admit a factual allegation, he can deny the factual allegation, or he can state that does not have sufficient knowledge to either admit or deny the allegation.
If a defendants claims insufficient knowledge, he has to attach an affidavit of his lack of knowledge. Basically, a party can't just say that he doesn't know. He has to swear under oath that he doesn't know. The consequence for failure to attach the required affidavit is severe. The court can deem the allegations admitted where the defendant claimed insufficient knowledge.
A recent case explains the issue. I have copied the relevant portion of a case called Parkway Bank and Trust Company v. Korzen, 2013 IL App (1st) 130380.