Introduction Mishnah one is an introductory mishnah to the entire tractate. Besides mentioning oaths it mentions knowledge of impurity, a topic to be discussed throughout the remainder of the chapter.
Oaths are of two kinds, subdivided into four; There are two kinds of oaths of “expression” (category one above, in introduction) which are explicit in the Torah: 1) A person swears to do something such as eating; 2) A person swears to refrain from doing something. If a person takes such an oath and then breaks it, he will be obligated to bring a sacrifice to atone for his sin (see Leviticus 5:4-13). The types of oaths specifically referred to in the Torah are oaths regarding the future, that a person will or will not do something. The Sages added two more types of oaths: those taken with regards to the past: 1) A person swears that he did do something; 2) A person swears that he did not do something. This is what the mishnah means when it says that there are two types of oaths which are four.
The laws concerning the discovery of having contracted uncleanness are of two kinds, subdivided into four; There are two types of sins mentioned in the Torah with regards to one who becomes impure without realizing it. The first is one who is impure and then eats holy food, such as sacrifices or terumah (heave offering). The second is one who is impure and then enters the Temple. One who had intentionally committed one of these acts would be obligated for kareth (excommunication). Our mishnah teaches that one who unintentionally does one of these acts, and later realizes what he has done, is obligated to bring a sacrifice. The Sages added two other types of sins of this nature. 1) A person knew that he was impure but did not know that what he was eating was holy food; 2) A person knew that he was impure but entered the Temple without realizing that he was doing so. In both of these cases the person is again obligated for a sacrifice.
The laws concerning carrying on the Sabbath are of two kinds, subdivided into four. On the Sabbath it is forbidden to carry an object from the private domain to the public domain. The two primary categories of this prohibition are: 1) A person stands in the public domain and puts his hand into the private domain and brings something out; 2) A person stands in the private domain and puts something out into the public domain. The Sages taught that there are another two categories of forbidden carrying: 1) A person stands in the private domain and puts his hand out into the public domain and brings something in; 2) A person stands in the public domain and puts something from there into the private domain. These laws are discussed at greater length in Tractate Shabbath. They are only mentioned here due to the similarity in language with the first two sections.
The symptoms of negas are of two kinds, subdivided into four. A nega is a skin disorder, discussed at length in Leviticus 13. It is often referred to as leprosy but in truth we do not know precisely what disease is referred to in the Torah. Our mishnah teaches that there are two types of negas mentioned specifically in the Torah: 1) A swelling; 2) A discoloration. The Sages added two more types of negas, both of which are learned from the word “rash” mentioned in the verse. These laws are discussed at greater length in Tractate Negaim.