Many of the Torah’s prohibitions involve a specific measure of activity that may not be performed or quantity of food that may not be consumed. According to Torah law no punishment is administered if less than that measure is performed or quantity consumed. The amora’im debated whether performing or consuming less than the required measure is prohibited by the Torah or is only prohibited by rabbinic decree in order to prevent a person from transgressing the prohibition itself. (In this context the term “half” is not to be taken literally, and implies any amount less than the forbidden measure or quantity.)
If one ate one piece of forbidden fat and then ate another piece of forbidden fat in one lapse of awareness, he is liable to bring only one sin-offering.
Rav Huna explains that the Mishna teaches this rule, which seems fairly obvious, because we are dealing with a unique case, where the person committing the transgression had a “period of awareness” between eating two half olive-bulks. After eating the first half of an olive-bulk, he became aware that he had eaten food that was prohibited. Then he became unaware again and ate the second half of an olive-bulk. Although, with regard to sacrifices, awareness usually serves as a line of demarcation between unwitting transgressions performed prior to the period of awareness and unwitting transgressions performed thereafter, Rav Huna explains that the Mishna is in accordance with the opinion of Rabban Gamliel who rules “There is no awareness for half a measure.” Since one is not liable to bring a sacrifice for half a measure, the fact that one became aware between consumption of the two halves of an olive-bulk is of no significance and does not demarcate between the two half-measures with regard to liability to bring a sin-offering.
The essential question regarding half a measure, which is the subject of an amoraic dispute, is to what extent is half a measure a significant amount by Torah law? Do Torah prohibitions prohibit any amount of the item, and it is only liability that requires the definition of legal parameters? Or, does the prohibition itself apply only to the requisite measure, i.e., anything less than that measure is permitted by Torah law and prohibited only by rabbinic decree? The question of whether awareness of half a measure is significant is based on the same dispute.