Rabbi Mordecai Kornfeld,
In the middle of Yoma 11a the Gemara cites a breisa that a "private" mezuzah needs checking twice in seven years and a "public" mezuzah needs checking twice in fifty years. From a Torah perspective, we can rely on the chazaka that the mezuzah is good, but that the rabbanan said you cannot just rely on the chazaka and that we need to check with some periodicity. I am wondering what is the sevara behind the frequency of private - twice in seven years? I talked with a number of rabbanim about this and most believe that the number is not based on probabilities and that the rabbanan did not base it on empirical evidence (that is not how chazal work). So that leaves "hinting" from a passuk or some sort of kabbalistic / mystic type thing. Do you have any ideas?
I must agree with the Rabanim that it is not based on probabilities - at least not with probabilities that are associated specifically with Mezuzos. We find that this figure of "twice in seven years" comes up in many contexts. For example, the attic of the Beis ha'Mikdash was checked twice every seven years (according to one opinion) to see if it needed any repairs (Pesachim 86a). Also, the four-letter name of HaShem was transmitted from Rebbe to Talmid twice every seven years (Kidushin 71a). From these examples we see that this figure is not particular to the rate of degeneration of Mezuzos on private doorways.
I'm afraid I don't know of any mystical or Kabalistic reasons for this figure (though I'm sure they exist), but I could offer a practical explanation. When I hear the figure "twice in seven years", I think of Kri'as ha'Torah on Monday and Thursday. Chazal did not want us to go more than three days without hearing the Torah (Rambam, Hilchos Tefilah, 12:1). (Even though they instituted three Torah readings per week, we could say that Shabbos is significantly different than the other days of the week, so essentially what they were doing was instituting Kri'as ha'Torah twice in the six working days to supplement the Kri'ah of Shabbos). The seven-year Shemitah cycle parallels the seven-day week. Just as Kri'as ha'Torah is never far away in our weekly schedule, Chazal wanted certain things like checking Mezuzos to be fixed in our seven-year cycle so they would never be too far away. (Rav Chaim Kanievsky says that "twice in seven years" means spacing them evenly over the seven years.)
We find that the Torah itself instituted something "twice in seven years" - Ma'aser Ani (tithes for the poor) which is given in the third and sixth years of the Shemitah cycle (see Pirkei Avos 5:12). Perhaps Chazal were following their general principal of Tikun k'Ein de'Oraisa (Rabinic laws are modeled after Torah laws) when they instituted "twice in seven years" for all of the above things.
R' Sigler and Rabbi Kornfeld,
Thank you for providing an answer to my question. I appreciate the work the Kollel Iyun Hadaf does in propagating Torah, and am doing my part (monetarily) to help you continue your activities.
One of the rabbanim I have discussed this topic with is Rabbi Aaron Gross here in Milwaukee; you can read his comments to your answer in the paragraph below. In light of what Rabbi Shlomo Zalman Auerbach seems to be saying, does this mean that statistical probabilities play a role in halachic matters (both in the world of mezuzahs but also other matters)?
I like the answer. However, I direct you to sefer Halichos Shlomo (R' Shlomo Zalman, ZT"L) vol. 1, ch. 4, last note in the chapter, where he is discussing checking teffilin and mezuzos; that if one affixes a mezuza in a glass case, i.e. that the mezuza itself is not touching the wall, then it need not be checked twice in seven years. From his general discussion there it seems that with teffilin (today's workmanship/product is much better than ever) and mezuzos one is better off leaving it alone when there is no reason to suspect any problem. The implication is that it is indeed a matter of probability.
I apologize for taking so long to reply to your reply, and I also thank for the interesting reference.
I agree with you that it is a "matter of probablility", but I don't think that that probability is roughly once in three and a half years (or even once in twenty five years). Rather, I think that Chazal decided that the probability of deterioration was greater than the probability of causing problems by opening it up to check it (unlike Mezuzos in glass cases or today's Tefilin) and therefore required checking it. How often? For that, they chose their all-purpose rate of twice in seven years (for reasons that I enumerated in my first reply).