Our Gemara quotes a baraita that compares and contrasts a neder and a shevua. According to the baraita, each of them has a strength that the other does not possess. Specifically, nedarim can be made on mitzvot, as well as on things that are permissible, while shevuot are limited and can be made only on permissible things. On the other hand, shevuot can take effect even on things that have no physical characteristics (davar she-en bo mamash) while nedarim can only take effect on things that have physical characteristics (davar she-yesh bo mamash).
Many of the rishonim explain that shevuot can take effect even on a davar she-en bo mamash because a shevua is accepted by the person on himself – the terminology that is used is harei alay (“I accept upon myself”), rather than the wording of a neder, which is harei zo (“That thing should be forbidden”) – and the person himself is a davar she-yesh bo mamash. The Ritva suggests a more basic approach, which we have already encountered (see Massekhet Nedarim, daf 2). In a neder, the statement made by the person takes effect on the object – e.g., when a person takes a vow not to eat a certain food, the food is now forbidden. A shevua, on the other hand, takes effect on the person, so that now there is a prohibition on the person to eat the food. It is the difference between an issur heftza – a prohibition on the object – and an issur gavra – a prohibition on the person.
In our Gemara, the difficulty is how to understand the Mishna which appears to permit a person to make a neder forbidding him to speak to someone, to help him or to walk with him. Given the fact that a neder must take effect on a davar she-yesh bo mamash, how are we to understand these rulings? Rav Yehuda explains that we must interpret his statement as meaning that he forbids his mouth to speak, his hands to support and his legs to walk. Thus the neder applies to specific parts of the body rather than to the activities.