כתובות מג:
"תרי זימני הוי חזקה"
Our Gemara is referring to a מחלוקת regarding the basic parameters that create a חזקה. In יבמות [סד:] the Gemara brings that רבי holds, if one gave a Bris to 2 of his sons & they died, he should not be מל the third son. רשב"ג says he should be מל the third & if he dies, then he should not be מל the fourth. Similarly, they hold the same thing by an אשה קטלנית & by a שור המועד. Is it after 2 times or after 3, that creates a חזקה? [קובץ הערות, הוספות אות ב'] ר' אלחנן explains that less than 3 times (or 2 depends on שיטה) we say, that which the תינוק died was only a מקרה (coincidence) & this מקרה is not relevant to the other brothers. But if this happened to 3 brothers (or 2) than we don’t say this is 3 separate happenstances because a מקרה doesn’t happen often & something that happens often can’t be considered a מקרה. Consequently, we must assume there is a common סבה that is responsible for these outcomes.
The אחרונים kler if a חזקה is a "בירור למפרע", or is it a "סימן". For example, when a שור gores & he is now classified as a שור המועד, is it because the previous 3 times (רשב"ג) or 2 times (רבי), are now a בירור to us that he always was a מועד. Or, (עיין רמב"ם נזקי ממון פ"א,ה"ד] does it mean that after these multiple goring’s his טבע has now changed & he has now become a gorer?
The [או"ח ס' קיד, סי' ח'] רמ"א says that for the first 30 days after שמיני עצרת if one is in doubt if he inserted מוריד הגשם he should assume that he didn’t & didn’t insert anything (minhag Ashkenaz doesn’t say מוריד הטל) & must repeat שמונה עשרה. After 30 days of saying גשם he may rely on a חזקה that he said גשם. The טור brings that the מהר"ם מרוטנבורג wouldn’t wait for 30 days to elapse but would say from אתה גיבור through מוריד הגשם 90 times in a row on שמיני עצרת (equivalent to 30 days x 3 tefillos a day = 90). This way he had his חזקה right away. His proof that this works is from [מס' ב"ק כד.] שור המועד there the Gemara says "ריחק נגיחותיו חייב, קירב נגיחותיו לא כ"ש"? I.e.: If an ox gores once a day for 3 days straight he is classified as a מועד, so certainly if he gores 3 times in one day, he’s a מועד. So too by us says the מהר"ם, if saying גשם 90 times over 30 days establishes a chazaka, then ק"ו saying גשם 90 times in one day should establish a chazaka. The טור brings ר' פרץ ז"ל that argues with this comparison of the מהר"ם & says saying it 90 times in one shot would not establish a chazakah.
The סטייפלר [קהילת יעקב ב"ק, ס' כט] explains that the מחלוקת between them lies in the fundamental understanding of ג' פעמים by a שור המועד. Is the יסוד that after we see an ox gore for 3 times, that’s a proof & a סימן that his nature was always a מועד & now we know for sure? Or does the goring of 3 times, now turn his nature [טבע] into a gorer, even though he wasn’t one before that? The מהר"ם learns that the 3 times “creates” his nature, so this formula will apply to creating a "הרגל לשון" for גשם. ר' פרץ learns that the 3 times is only a proof to what the ox always was, but doesn’t create a nature, thus this has no relevance to גשם, where we are trying to create a habit.