Berachos, Mishnah 3. Before us is another dispute between Beis Shammai and Beis Hillel concerning conduct at the meal:
"Beis Shammai omrim: mekaneiach yadav bamapah u'manichah al hashulchan; u'Beis Hillel omrim: al hakeses" - after washing the hands, a person dries them with a napkin. According to Beis Shammai he should place the napkin on the table, and according to Beis Hillel it should not be placed on the table but on the keses - the cushion lying next to the table.
The root of the dispute:
The basis of the dispute lies in the question of whether one may use a table that has a second degree of tumah.
The view of Beis Shammai:
Beis Shammai hold that it is forbidden to use a table that has a second degree of tumah, out of concern that Terumah might be eaten on that table - and Terumah that comes into contact with something of second-degree tumah becomes third degree and is disqualified. Since it is forbidden to use a table that is tamei, it follows that the table is tahor, and there is no concern that the water on the napkin will become tamei when the napkin is placed on it.
The cushion, by contrast, may be used even if it is tamei, since it does not lie on the table and does not come into direct contact with the food, and people are not particular about it. Therefore, if a person places the napkin he has used on the cushion, the napkin and the water in it will become tamei - and so he places it on the table.
The view of Beis Hillel:
Beis Hillel hold that one may use a table that has a second degree of tumah. And what of the concern that Terumah eaten on it will be disqualified? That concern applies to kohanim alone, and the kohanim - as we have seen - are meticulous in their conduct and will be careful not to use such a table; but an ordinary person may use a table that is tamei.
That being so, one may not place on the table the wet napkin with which he dried his hands after washing: the water in it will become tamei from the table and will become a first degree of tumah - for liquids always become a first degree of tumah - and that first degree is liable to render all the food on the table tamei, making it a second degree of tumah, so that he ends up eating food that is tamei.
True, placing the napkin on the cushion lands us in a different problem, for the cushion is easily rendered tamei, and it turns out that the napkin is resting on something tamei. If so, what have we gained?
The gain is that the food does not become tamei, for the damage in each of the two options is fundamentally different:
Placing it on the table: the table is tamei, the liquid in the napkin becomes tamei and renders the food on the table tamei - and the food is tamei by Torah law.
Placing it on the cushion: the hands are liable to become tamei through contact with water that became a first degree of tumah because of the cushion - and tumah of the hands has no source in the Torah.
In summary: when a person is forced to choose between a table that is tamei and a cushion that is tamei, it is better that he place the napkin on the cushion that is tamei, whose entire damage is limited to tumah of the hands, which has no source in the Torah, and not on the table that is tamei, where placing it would lead to the food becoming tamei.