Parshat Tzav: Kotzker Rebbe – Vouching For Purity?

“V’HaBasar Asher Yiga B’Chol Tamei Lo Yei’achel”, the meat of a Korban that touches anything tamei may not be eaten (Tzav 7:19).  The Kotzker Rebbe was asked why if something

tamei touches something that is tahor, does the tahor become tamei and not vice versa?

He answers that when something is tamei we know for sure that it is tamei.  However something that we consider tahor, only means that we don’t know that it became tamei.  Can anyone really vouch with certainty that something is truly 100% pure?