Ohalos, Chapter 3, Mishnah 6. Our Mishnah gives us the measurements of the openings of a house: how wide a doorway or window must be in order for it to serve as the recognized exit route of tumah, and how wide an opening must be in order for tumah to travel from one room into another. Let us first translate the words, and then go back and explain what stands behind them.
Translating the Mishnah
"Kezayis min hameis, piskho betefach." Where the house contains no more than a kezayis taken from a meis, the opening under discussion must measure a tefach. "Vehameis, piskho bearba tefachim." Where an entire meis lies within, the opening must measure four tefachim. To what end? "Lehatzil hatumah al hapesachim," to spare the other openings, so that the tumah should not be reckoned as liable to depart through any of the remaining doors and windows.
"Aval lehotzi hatumah bepoyseach tefach." When it comes to tumah traveling out of one area into another, say from a room in the house into the room beside it, an opening of one tefach already does the job.
"Godol mikezayis, kemeis." A portion of a meis that exceeds the size of a kezayis is treated for this halacha as though it were an entire body. "Reb Yosi omer, hashidrah vehagulgoyles kemeis." Reb Yosi adds that a backbone lying there, or a skull, likewise ranks as a full meis.
The Background: Every Doorway Is a Possible Exit
To understand what the Mishnah is measuring, we need a principle that we will meet again later on, in the third Mishnah of the seventh perek, and in many Mishnayos after it. When a meis lies in a house, all of the openings of that house, every doorway and every window, are liable to become tamei.
Picture yourself standing in a doorway from the outside, with the door itself shut in front of you. If you were to take one step forward your face would meet the closed door. But look upward: you are standing directly beneath the mashkof, the lintel, the upper beam of the doorframe. In that position you can become tamei from the meis lying inside, even though the door in front of you is closed. Keilim placed there can likewise become tamei.
Why should this be? Because of the rule of "soif hatumah latzeis," the tumah is destined to go out. The meis will not remain in the house forever; it must eventually be carried out. As long as no particular exit has been settled upon, every opening of the house is regarded as a possible route by which the tumah will leave, and therefore whatever stands in any of those openings is affected.
This spread across all the openings can be brought to a halt in two ways. One is when every door and window is sealed apart from a single one; since that is the only passage available, it is automatically the route by which the tumah will depart, so it alone becomes tamei while the rest stay tahor. The other is when a person resolves in his mind that the meis will be taken out through one specific opening; that opening becomes tamei and the others are, as it were, let off and remain tahor.
The Bartenura tells us that this halacha, that all the openings become tamei, is a gezeirah of the Chachomim. There are others who hold that it is a halacha leMoshe miSinai.
One more piece of background: when a meis is in a house, the tumah fills the entire house, but it does not simply pass into a different house. It can move from one house to the next only when there is an open window or opening between them. This is the point alluded to in the closing Mishnah of maseches Shabbos, and we will see the details of these cases further on, be'ezras Hashem.
The Halacha of Our Mishnah
Now the question of our Mishnah becomes clear. I want to designate one particular opening as the route by which the tumah will leave, whether because it is the only accessible one or because I have decided upon it, so that the remaining openings should stay tahor. How large must that opening be for this to work?
"Kezayis min hameis, piskho betefach." Where the house holds nothing more than a kezayis taken from a meis, the opening chosen for this purpose has to be a tefach by a tefach. At that size it qualifies as a workable exit, and the rest of the doors and windows are thereby kept tahor.
"Vehameis, piskho bearba tefachim." If a whole meis lies in the house, one tefach is not enough. Only an opening of four tefachim by four tefachim can be designated as the route of departure, and only then may we say that what stands in that opening is tamei while everything else is tahor.
"Aval lehotzi hatumah bepoyseach tefach." At this point the Mishnah takes up a separate question, and here the ruling is more severe. Suppose my interest is not in which opening protects its fellows, but in whether tumah from a meis lying in one room can pass through an open doorway or window and affect people or keilim standing in the room alongside. For that, one square tefach is all that is required. Even where a whole meis is involved, a gap of a tefach by a tefach lets the tumah cross over and render whatever stands opposite tamei.
"Godol mikezayis, kemeis." A piece of a meis that is bigger than a kezayis, even though it is not a whole body, carries the din of a whole meis in this regard. To establish that the tumah is leaving specifically through one window or one doorway, that opening would need to be four tefachim by four tefachim.
"Reb Yosi omer, hashidrah vehagulgoyles kemeis." Reb Yosi holds that a spinal column or a skull is likewise judged as a full meis, and here too the designated opening would have to measure four by four tefachim.
Reviewing What We Have Learned
A meis in a house, or even so much as a kezayis of one, renders all the openings of that house tamei. A shut door does not protect the space beneath its lintel: someone standing outside, right up against the closed door and under the doorframe, is susceptible to tumah, and keilim set there are as well. The reason is the rule of soif hatumah latzeis: every doorway counts as a possible route out for the body. That holds unless all the rest of the openings are sealed so that just one is usable, or unless a person has made up his mind to carry the meis out through one specific doorway.
We also learned that a meis in one house can render the neighboring house tamei when an open window connects them, and the fuller details will follow later, be'ezras Hashem.
And the Mishnah before us supplied the measurements. Where the house contains merely a kezayis taken from a meis, an opening of a tefach square, once it has been designated, suffices to keep the other doorways tahor. Where a full meis lies there, the figure is four tefachim by four tefachim. But for tumah to move across from one room to its neighbor, a single square tefach already does the job. A fragment exceeding the size of a kezayis carries the din of a meis and needs the four by four. And according to Reb Yosi, a backbone or a skull is judged as a meis as well, so there too the opening must be four tefachim by four.
A Thought to Take Along
I once heard a shmuess from one of my rebbeim resting on exactly this idea. There are times when a person lands in a matzav where no way forward is visible at all. It seems as if everything has turned against him, every door has been locked, and he is boxed in with nowhere to turn. Then he resolves: no, the answer is with Hashem. Hashem prepares the refuah before He sends the makah. I will come through this. Another Purim will come, another Pesach, another Shavuos; my whole life is still in front of me. In what way will it happen? Hashem, that is Yours to know. Hashem, I trust that You have already prepared the way out for me.
The moment a person settles that in his heart, the exit exists, and everything else, all the imagined locked doors, becomes tahor. Hashem always creates a doorway through which our troubles can leave, even when we do not yet know which doorway it will be.