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1. The Mishnah discusses two ways in which one can transgress the prohibition of carrying on Shabbos.
2. When a person transgresses the prohibition of carrying on Shabbos, only the person who actively brings the object into another domain transgresses.
3. The Mishnah continues to discuss the two other cases in which one is not obligated to bring a Chatas for carrying on Shabbos.
4. The Gemara quotes three other places where the Mishnayos states "two that are four."
5. The Mishnah here says "two that are four inside.," in contrast to the Mishnah in Shevuos that says merely "two that are four."
A BIT MORE
1. For example, a person standing inside a house can transgress carrying (according to Torah law) in two ways: through bringing an object from a public domain (from "the hand of a poor man" standing outside) into his house, or by taking an object from his house and carrying it out to the public domain (by "placing it in the hand of the poor man" standing outside).
2. When a homeowner takes an object from the hand of a poor person and brings it into his house, the poor person is exempt from any transgression of carrying on Shabbos.
3. When the Mishnah says "two that are four," it refers to two ways of transgressing for which one must bring a Korban Chatas (as in #1), and two ways for which one is not obligated to bring a Korban Chatas. For example, if a poor person outside stretched his hand out of the public domain into a house and the homeowner inside took what was in the poor person's hand, neither is obligated to bring a Chatas.
4. These laws involve: swearing falsely, knowing about Tum'ah when entering the Mikdash and eating Kodesh, and types of Tzara'as.
5. This is because the Mishnah here is in Maseches Shabbos, where this topic is discussed at length. In contrast, in Maseches Shevuos the topic of Shabbos is not the primary topic.
Next Daf Index to Revach for Maseches Shabbos