As I Heard Them, Vol. 1, pp. 89-90, no. 22
A young man once told the Tzemach Tzedek that since his city, Liepli, has apikorsim (heretics) he has to fulfill the directive, Da ma shetashiv la’apikores – Know what to answer a heretic.27 “What should I say?” he asked the Rebbe.
“The apikorsim who are walking around there,” answered the Rebbe, “you must avoid any contact with. But for your own inner apikores the antidote is to ‘Da ma,’ Know ‘what’ — which refers to Chochmah (wisdom) and bittul (self-nullification).”
27. Pirkei Avos 2:14.