“…But he does not know, he is guilty and shall bear his transgression…” – Vayikra 5:17

וְלֹא יָדַע וְאָשֵׁם וְנָשָׂא עֲוֹנו – ויקרא ה, יז

Why is it necessary to atone for transgressions that were committed unknowingly, especially if one is doubtful if he transgressed at all?

Chassidus explains that although the actual transgression was not committed intentionally, yet its very occurrence is indicative of an internal spiritual weakness. For what causes a person to succumb to sin inadvertently is his instinctive draw and desire for such behavior.  He is therefore guilty for his choices in the past that caused this attraction to the impermissible, even if only subconsciously, which then manifests itself in an actual transgression. Conversely, the verse in Mishlei (12:21) states, “No corruption shall chance upon the righteous.” The righteous person craves only G-dliness, therefore he will not chance upon sin even unintentionally.

This explains why the classic example of doubtful transgression brought in the Talmud is a doubt regarding a piece of animal fat. As quoted in Rashi on the verse cited above, “a piece of prohibited animal fat and a piece of permissible animal fat lay before someone, and, thinking that either was permissible, he ate one. He was later informed that one of those pieces was prohibited fat. As he doesn’t know whether the one that he had eaten was indeed the prohibited one, he brings a ‘pending guilt-offering’.”

In light of the above explanation regarding the cause of unintentional sin, the Talmud’s choice of illustration is precise. Fat represents lusciousness and pleasure. Accordingly, the doubt whether one has indulged in forbidden fat or not is essentially the question that lies behind every possibility of unintentional sin: have you been drawn to lust the prohibited or do you delight exclusively in the holy and permissible?

—Likkutei Sichos vol. 3, pp. 944-946