In the third perek (3:7) we are introduced to the concept of derech hatumah latzet - "it is the way of tumah to leave". To explain, tumat ha'met (tumah originating for a corpse) spreads making everything under the same ohel (covering) tameh. The tumah can spread from one ohel to the next one beside it, if there is a gap of at least a tephach by tepchach in the partition that separates between the two. The exception to this is the case of derech hatumah latzet.
The case that illustrates this point is the biv (pipework) that is running beneath a house. Its cross-sectional area is a tephach by tephach such that it is defined as an ohel. Both ends of the biv however are less than a tephach such that one would not expect the tumat hamet to spread from one to the other. The Mishnah teaches that if a kezayit from a met is found in the house, then anything found in the biv is indeed tahor. If however the tumah is found in the biv, then the contents of the house is tameh. The reason is that in this case, since the biv's end is less than a tephach the way tumah would "leave" is through the house itself. We shall try to understand this principle.
It is important to note the this concept of derech hatumah latzet is raised in other mishnayot also. The first Mishnah of the next perek discusses a case of a large cupboard found in the house. Based on this same principle, if the tumah is in the house, the contents of the cupboard are tahor, while if the tumah is in the cupboard, the contents of the house is tameh. Similarly, next week we will learn about the case (4:2) of a large chest whose drawers are a tephach deep but opening to the room is less than a tephach. Once again, if the tumah is in the house, the contents of the drawer are tahor, while if the tumah is in the drawer, the contents of the house is tameh. In that case, R' Yossi disagrees since the tumah can be removed from the drawer in pieces less than a tephach in size.
The Mishnah Achrona (4:2) questions the necessity of all these cases if they are seemingly based on the same principle. Furthermore, why is it only in the last case that R' Yossi argues?
The Mishnah Achrona explains that each case is different and necessary. In our case, the biv is dug in the ground under the house. The reason why the house is tameh in this cases, is since there is no opening at either end the size of a tephach, the biv is equivalent to a kever satum (a sealed grave). In that case a kever satum makes all adjoining ohalot (above, below or beside) tameh. The Mishnah Achrona (3:7) explains that this is different to tumah that is simple buried, where it makes tameh anything that is directly above it tameh. In other words, our case is a special case of derech hatumah latzet -- and even R' Yossi would agree that the house would become tameh.
The Mishnah Achrona continues that ordinarily there are two different ways to understand derech hatumah latzet. The Rama understands that since the tumah will eventually be removed via the room, it is viewed as if it is already in the room. Alternatively, he cites the Mahari MiTrani, that it is deter one from using that area for taharot, out of concern that the tumah might be removed at the same time that the room is being used, or that the cupboard might accidently be opened.
Based on this, the Mishnah Achrona explains that R' Yossi would only argue in the case of the drawer. According to both understandings of derech hatumah latzet there is no reason for concern. Firstly, since the tumah can be removed piecemeal, it is not a certainty that the tumah will eventually transit through the room. Secondly, since the opening of the drawer is less than a tephach, even if it was opened, the opening would not be large enough for it to spread into the room.
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