Tafasta meruba lo tafasta (Hebrew: תפשת מרובה לא תפשת, "If you have seized a lot, you have not seized") is a Talmudic idiom used to express the idea that when it is possible to take a particular law from two different sources, it should be taken from the narrower of the two, in order to stay on the safe side and avoid making assumptions about which is correct. It is akin to the logical concept of "proving too much". This will be illustrated with an example below.
Contents
Terminology and meaning
The entire phrase actually includes both a positive and a negative expression: תפשת מרובה לא תפשת תפשת מועט תפשת - "If you have seized a lot, you have not seized; if you have seized a little, you have seized." The general meaning is that an over-ambitious claim defeats itself: the intended analogy is to one who grabs more than he can hold.
In Yoma 80a, the mythical egg of the enormous Bar-Yochani bird is provided as an example of Tafasta meruba lo tafasta: "ואימא ביצת בר יוכני תפסת מרובה לא תפסת תפסת מועט תפסת ואימא ביעתא דציפורתא דזוטר טובא"
Explanation
The use of this concept is best explained with an example from the Talmud:
In comparison to the holidays of Passover and Sukkot, Shavuot is a relatively short Jewish holiday. This posed a problem in the times of the Holy Temple because there simply would not be enough time for the holiday sacrificial offerings of the entire nation to be sacrificed; Passover and Sukkot were seven and eight days, respectively, while Shavuot was but a single day. The Talmud explains that there was an extended period of time, referred to as tashlumin (period during which the remaining sacrifices could be completed), that was appended to Shavuot to make up for the relative shortness of the holiday.
The question is: for how long can the Shavuot holiday offerings be brought? With the presence of an extra mention of both Passover and Sukkot, both could be used to determine an equivalent period of time—comparing Shavuot to Passover would yield a seven-day period, while comparing it to Sukkot would yield an eight-day period, and there is nothing pressing one choice over another.
The Talmud concludes that Shavuot is compared to Passover to yield a comparable seven-day period to complete the sacrificial offerings. Why not compare Shavuot to Sukkot and gain an extra day? So the rule of tafasta meruba lo tafasta comes to show us that we cannot take more time than we are sure is allowed. Either way we're following the precedent and Shavuot will last at least seven days, but it is only by presuming, without grounds, that it should be compared to Sukkot that we can get the eighth day. In order to stay on the safe side, we must use the limit of seven days from Passover.