What is the definition of rosho verubo? does rubo mean the majority of the torso alone, or does it include the legs?
Furthermore, do we measure the majority of the body without the head-- or perhaps it means that rosho+rubo must equal a majority?
Shmuel Globus, Jerusalem
The Terumas ha'Deshen (RavYisrael Isserlin) explains that according to Tosfos (Shabbos 92a) a person's body is three Amos or eighteen Tefachim (approximately 150 cm) excluding the head. It is clear from Tosfos that this includes the legs. The Terumas HaDeshen then proceeds to explain that from here we see that Rosho v'Rubo is more than ten Tefachim. (He obviously assumed that a person's head is more than two Tefachim.) We see from here that Rosho v'Rubo is half the persons whole body including head, torso and legs.
The basis for the Halachah that we consider his entirety to be in the domain where his Rosho v'Rubo are, is the rule of the Rubo k'Kulo. This means that we consider the minority to be insignificant and consider his majority to represent his entirety. When we say Rosho, we do not require all his head as here also Rubo of the head would suffice.The reason the Tana specifically mentions Rosho and did not just say we need Rubo of his whole body is because we would not consider him to be in a certain place through the rule of Rubo k'Kulo if his head would be in a different domain since his head is an important part of his body and cannot be considered insignificant.
Accordingly, as long as the majority of a person's head and the majority of his body are in the same domain we say he is considered to be in that domain. There is therefore no need for Rov of his body besides the head to be in that domain.
D. Freedman