Oaths are of two [types] which are [in reality] four. Awareness of tumah is of two [types] which are [in reality] four. Transfers on the Sab-bath are of two [types] which are [in reality] four. Appearances of the [tzaraas] affliction are of two [types] which are [in reality] four.

R' Chanania ben Akashia says: The Holy One, Blessed is He, wished to confer merit upon Israel; therefore He gave them Torah and mitzvos in abundance, as it is said: Hashem desired for the sake of its [Israel's] righteousness, that the Torah be expanded and strengthened (Isaiah 42:21).