When discussing the prohibition against eating forbidden foods, how much must a person eat in order to be held liable?
According to the Mishnah (13a) there is a difference of opinion on this question. With regard to tevel – untithed produce that is forbidden – Rabbi Shimon believes that eating even a tiny amount would be enough for someone to be held liable; the Hakhamim rule that a person must eat at least a ke–zayit – an amount of food equivalent to an olive-size. By way of explanation, the Mishnah brings a conversation between the Sages. Rabbi Shimon argued “Do you not agree that someone who eats a tiny ant will be held liable?” To which the Hakhamim answered “A tiny ant is ke-beriyato” – it is a full creature. Rabbi Shimon replied that a single piece of grain is also ke-beriyato.
Our Gemara explains that the point of disagreement between the Sages is dependant on the question of whether only a living creature would be considered important enough for its statues as ke-beriyato to be significant. The Gemara concludes the explanation of Rabbi Shimon’s position by explaining that according to him a person would be liable to receive the punishment of malkot (lashes) even if he ate a small amount; were he to have done the forbidden act accidentally and have been required to bring a sin-offering, he would only be obligated in a sacrifice if he ate the amount of a ke-zayit.
Rashi explains this last ruling – that a sacrifice will only be brought if a ke-zayit worth was eaten – as being based on a halakhah le-Moshe mi-Sinai – an oral tradition handed to Moses on Mount Sinai. The Ritva, however, offers an explanation. If someone is liable to receive malkot, he clearly has eaten the forbidden food on purpose. In that case, his decision to eat the food shows that in his mind the food is important, even if it is a very small amount – for which he will be punished. If he ate the forbidden food by accident, however, then the food will only be important enough to bring a sacrifice if there is an objectively significant amount – a ke-zayit.