1) "Umdena" means "estimation."

2) Let us look at an example of Umdena in Maseches Bava Basra to get an idea of what it means.

The Mishnah (27b) states that if a tree leans over in the direction of the public domain, one may cut off enough of the lower branches which will allow a camel and its rider to pass under the tree. The question is, why is it sufficient to cut off the branches? Why does one not cut down the entire tree, since it represents a public disturbance because the branches will inevitably grow back again? (See Tosfos Yom Tov who says that the question is why does one not cut down the entire tree?)

The Gemara answers this question when it says, "Who is the Tana who follows the 'Umdena d'Hashta,' the present Umdena?"

The Gemara is indicating that the reason why it is not necessary to cut down the entire tree is that there is an Umdena; we estimate that people are happy enough with looking at the situation now. At the present moment, since we have cut off the branches, the tree is not disturbing anyone. Even though later on the branches will grow back and cause trouble, we estimate that people are satisfied with the situation now, and it is not justifiable to cut down the tree. When the branches do start growing back, "Kama Kama Ka Kayitz Lei," which Rashi (DH Kama) explains to mean that whenever a branch grows back we will cut it off and this is sufficient in order to control the growth of the tree.

3) Now we will look at an example of Chazakah in Maseches Bava Basra. On 5b the Gemara states that there is a Chazakah that a person does not pay back a debt before the date that was agreed upon for him to pay. This seems to be quite a different concept to that of Umdena. Chazakah here is a description of the nature of people. The nature of most people is that they wait until the last possible moment (or possibly even later) before paying back. This is different from the Umdena on 27b where we have an estimation of what people find to be disturbing behavior in the Reshus ha'Rabim.

4) To better understand what an Umdena is, let us see more examples. Let us go to Bava Basra 146b where the Gemara asks, "Who is the Tana who follows an Umdena? Rav Nachman answered, 'it is Rebbi Shimon ben Menasya.'"

(It is worth noting that we can learn from the fact that the Gemara on 27b, which we saw above, and the Gemara on 146b both ask, "Who is the Tana who follows Umdena?" that the legal concept of an Umdena is not something which is agreed upon by everyone, by only by certain Tana'im. Chazakah is different, since the Gemara in Chulin 10b asks, "What is the source for what the Rabanan said, 'Leave the matter on its Chazakah'?" which suggests that the concept of Chazakah is universally accepted.)

Rebbi Shimon ben Menasya discusses a scenario where Reuven's son went overseas and Reuven heard that his son had died. Reuven then wrote a command leaving all of his possessions to someone else. Afterwards it transpired that his son was in fact still alive. Rebbi Shimon ben Menasya said that the gift he gave to the other party no longer applies, because there is an Umdena that if he would have known that his son was still alive he would not have given all his property to someone else. Even though he did not write that the reason he gave his property to another person is because he heard that his son died, we still estimate that this must be the reason he did so, because there is an Umdena that a person does not forget about his son when dividing his estate.

5) I think we are already getting the idea that an Umdena relates to the way we understand a person's intentions in a specific case, while a Chazakah relates to the more general behavior of people. The Chazakah mentioned on 5b relates to every loan made in the world, and tells us when the average person is expected to return the money. The Umdena on 27b relates to a specific case of a tree hanging over the main road, and on 146b to a fairly unusual case of a person who thought his son had died.

6) I suspect that there may not be a hard and fast rule about which is stronger, an Umdena or a Chazakah, but it depends on each individual case, especially as there are different levels of Umdena. The Gemara (end of 146b) calls the case of the father who heard that his son had died an "Umdena d'Mukach," it is a well-proven Umdena. The Rashbam (end of 146b) explains that when the son was alive the father did not give the gift to others, so there is an especially strong reason to follow such an Umdena. Possibly we could say that an Umdena d'Mukach is often stronger than a Chazakah.

7) Here are examples of Umdena and Chazakah in capital cases:

a) The Gemara (Sanhedrin 37b) gives an extreme example of Umdena in a life and death case. This starts in the Mishnah (Sanhedrin 37a) which states that the witnesses for capital cases are threatened and told: "Are you possibly giving your testimony only from 'Omed,' estimation?" ("Omed" is the same as "Umdena.") It is not sufficient if the Edim only estimate what happened without clear knowledege.

b) The Gemara (37b) cites a Beraisa in which Shimon ben Shetach said that he once saw someone chasing his victim into a ruin. Shimon ben Shetach ran after the pursuer and saw him holding a sword dripping with blood. The murder victim was convulsing. Shimon ben Shetach said that the wicked murderer cannot be put to death by the Sanhedrin since there were not two witnesses who saw the crime with their own eyes.

c) The above Umdena would seem to be an "Umdena d'Mukach," a well-proven Umdena (see 6 above), but in capital cases it is not sufficient.

d) On the other hand, we learn in Kidushin 80a that Beis Din may administer Sekilah on the basis of a Chazakah. An example is a woman who came to Yerushalayim carrying a little baby on her shoulder. We did not know the family but we assumed that this was her son. She raised the boy and when he grew up, he had Bi'ah with the woman. They were put to death by the Sanhedrin for the sin of a son having relations with his mother. There was only a Chazakah that he was her son (since we assume that if she carries the baby on her shoulder and raises him that he is her son) but we have no proof for this. Even so, Beis Din put them to death. This seems to be a proof that while we do not apply the capital penalty for the Umdena of Shimon ben Shetach, we do give it for a Chazakah. This would be an example where a Chazakah is stronger than an Umdena.

8) Now let us explore: Is the "Lefichach" in the Mishnah (2a) a Chazakah or an Umdena?

a) The Mishnah states that since the two partners built the wall together, "Lefichach" if it falls down they divide the area and the stones equally. Tosfos (2a, end of DH Lefichach) writes that "Anan Sahadei" -- we are witnesses -- that one of the partners would not have built the wall on his own since he could have forced the partner according to the Din to build the wall with him.

b) We learn that Tosfos describes the assumption that they built the wall together as an "Anan Sahadei." Now I found, bs'd, another Tosfos that writes that an "Anan Sahadei" is an Umdena. In Kesubos 51a (end of DH mi'Merkaka'ei), Tosfos discusses the fact that according to the Din of the Gemara, there is only Shibud on land, but the Ge'onim made a Takanah that nowadays there is also a Shibud on movable items. Tosfos writes that nowadays "Anan Sahadei" that everyone gives to their daughters movable items, not land, when they marry them off. Tosfos writes that this is therefore like "Bei Amidnei." Even though Tosfos uses a slightly different word than "Umdena," when the Teshuvos Chavos Yair (#84) cites the words of Tosfos, he does use the word "Umdena." We therefore have a Tosfos which states that an "Anan Sahadei" is like an Umdena.

c) Therefore, we can say that when Tosfos in Bava Basra (2a) uses the phrase "Anan Sahadei," it also means an Umdena. However, I would add that an "Anan Sahadei" is not a standard Umdena; it is an Umdena d'Mukach (a compelling supposition), which is mentioned in Bava Basra (end of 146b) as I wrote above.

Dovid Bloom