Nedarim, Chapter Two, Mishnah Two
This mishnah compares vow formulas with oath formulas.
1) [If he says,] A korban should be what I do not eat of yours; By a korban! If I eat of yours; What I do not eat of yours should not be a korban to methe vow is not binding.
2) [If he says], An oath [that] I will not eat of yours; An oath that I eat of yours; No oath [that] I will not eat of yourshis oath is valid.
3) In these instances oaths are more stringent than vows.
a) There is [also] greater stringency in vows than in oaths.
b) How so?
c) If one says, Konam be the sukkah that I make,; The lulav that I take; The tefillin that I put on; as vows they are binding, but as oaths they are not, because one cannot swear to transgress the commandments.
Section one: I will explain each of these individually.
A korban should be what I do not eat of yours: This is an ineffective vow, because he is not forbidding anything which he will eat. The only thing that is forbidden to him is that which he will not eat in any case.
By a korban! If I eat of yours: This translation is based on an emendation to the mishnah, created by the Talmud. The mishnah itself states A korban should be what I eat from you. That should not be considered invalid formula. Therefore the Talmud suggests that the mishnah should read By a korban! If I eat of yours. He has not stated that the food should be forbidden, rather he has used the word korban to state that he will not eat from the other person. Such a formula is not valid in vows.
What I do not eat of yours should not be a korban to me: We might have assumed that this double negative implies that what he doesnt eat should not be a korban, but what he does eat should be a korban. However, this mishnah holds that we do not learn positive implications from negative statements. Saying that something will not be a korban does not mean that other things will.
Section two: All of the above formulas are valid if the word shevuah or oath is used instead of a vow. Again, I will explain them one at a time.
[If he says], An oath [that] I will not eat of yours: This is a normal oath formula is certainly valid.
An oath that I will eat of yours: It would seem that this statement should not cause him to be prohibited to eat from his fellows food. On the contrary it seems that he is saying that he will eat from his friend. The Talmud understands this to be referring to a case where Reuven is pressuring Shimon to eat at his house. Shimon says several times that he will not, and then finally blurts out An oath that I will eat of yours. The context of his saying makes it clear that Shimons intent was not to eat at Reuvens home. However, in other contexts this is not an oath formula that would cause a prohibition.
No oath [that] I will not eat of yours: We could deduce from here that although he has not made a prohibitive oath on that which he will not eat from his fellow, he has made a prohibitive oath on that which he will eat. Although above, in section one regarding vows we stated that we do not make positive deductions from negative statements, with regard to oaths we do.
Section three: The mishnah now points out that since the vows made in section one were invalid whereas the oaths were valid, oaths are in some senses stricter than vows.
However, in another matter vows are more strict. A person cannot take an oath not to observe a commandment, because he is already biblically obligated to observe the commandment. However, he could state that a given ritual object is forbidden to him with a konam vow. In such a case he is still obligated to perform the commandment. He just must perform it with another object.