OU TORAH Parshas Bechukosai Part 2 By Rav Moshe Twersky, HY"D

Regarding the fulfillment of a mitzvah, one needs to be aware if the mitzvah he is doing is d’Rabbanan or d’Oraysah (Rabbnicially mandated or Biblically mandated). If, when carrying out a mitzvah d’Oraysah, one mistakenly thinks that it is only d’Rabbanan, and he consciously intends to fulfill a mitzvah d’Rabbanan with his action, there is a strong argument to be made that doing so makes it that he has not fulfilled the mitzvah.