The halakhic legal system assigns highest weight to two witnesses. Based upon the pasuk, "Al pi shenayim eidim yakum davar," halakha affords eidim the strongest possible power. Both with regard to monetary matters as well as issurim, the testimony of two eidim overpowers every conceivable counter-force (migu, chazaka, rov etc).
This perspective might also lead to a limitation upon eidim. Indeed, in general eidim defeat any and every counter-force. However, in situations in which eidim face some inhibiting constraint, may they be assisted by some additional halakhic force in surmounting the problem they face. Can they benefit from the application of a migu or shevua the same way these forces may assist others? Or, do we maintain, that since they usually operate independently of any additional forces, even when vulnerable, they may not receive any outside assistance.
The three situations in halakha in which eidim would theoretically require assistance are:
1) A situation of 'trei u'trei' – contradictory testimonies. Even though Reuven and Shimon have testified, since their report has been contradicted by a subsequent group of witnesses, they are rendered ineffective.
2) A situation in which eidim are precluded from testifying because they possess some vested interest in the case, rendering them 'nogei'a be-davar.'
3) A situation in which they have already testified, thus preventing them from testifying a second time, due to the principle of 'kivan she-higid shuv eino chozer u-maggid' - eidim may testify about a particular event only once.
Can eidim overcome these constraints through the assistance of halakhic forces which are normally reserved for non-eidim?
Though no gemarot explicitly address this question, there are several gemarot which might provide some clues. The gemara in Ketubot (18b) considers the ability of signatories upon a contract to comment upon the validity of their signatures. The gemara raises the concern that kivan she-higid would disqualify them from further comment upon the shetar. Since they already signed, the document itself represents their first testimony; every document tacitly testifies to its own integrity. By signing it, eidim are considered as having testified that they were valid eidim at the point of issue of this shetar. Once they signed, they can no longer claim, "We were minors or relatives when we signed," because of kivan she-higid. Subsequently, though, the gemara does present a scenario under which they may render additional comments about their status when they signed. If these eidim could have subverted the shetar by denying the signatures altogether (and claiming they were entirely forged), they are believed to claim "these are indeed the signatures but we were minors or relatives." "Believe us," they may assert, "that these are indeed our signatures but we were minors at the point of issue, for if we were lying we could have posed a more absolute and powerful lie by claiming that these signatures are entirely forged." Namely, the concept of migu (multiple options towards achieving the same legal result) empowers them to issue a second testimony and surmount the obstacle of kivan she-higid. This gemara would seemingly suggest that assisted eidut may be employed.
The Ramban, however, in his comments to Bava Batra (31b) and Ketubot (19b), asserts that eidim must operate independently without the assistance of migu in overcoming legal constraints. The Ramban responds to a very compelling question which Tosafot raises in these two instances. A situation of trei u'trei concludes in a deadlock whereby neither group of eidim is fully believed. Tosafot inquire as to why the second group is not preferred since they enjoy a 'migu': Believe us as to the terms of the event, for if we wanted to disqualify the first group, we could have just as easily claimed that they were thieves. Had they personally attacked the first group, the second group would have triumphed, as former that group would have been defenseless to affirm their own validity (for in that matter they would not have qualified as witnesses but instead would have been considered ba'alei davar - subjects). Tosafot provide several answers to explain the ineffectiveness of migu under these circumstances. The Ramban, however, develops a fundamental position about assisted eidut. Since eidim are so powerful and generally independent, their eidut also has to be self-sufficient. If the second group of eidim require a migu to bolster their reliability against the contradictory group, they can no longer function as independent eidim. Though the Ramban does not provide a source for this 'self-sufficiency' principle, a similar Tosafot in Kiddushin (43b) claims that the pasuk describing eidim, implicitly demands self-sufficiency. By writing "al pi shenayim eidim yakum davar' (the verdict should be based upon the testimony of two witnesses) - the Torah implies that eidim must be believed solely upon their statement without any assistance. Supplying a migu to assist them subverts their identity as eidim.
How would the Ramban explain the gemara in Ketubot which endorses the use of migu to hurdle the obstacle of kivan she-higid? Though there are some options to explain the gemara without employing migu, the Ramban (in his comments to Bava Batra 31) does not adopt these approaches. Instead, he discriminates between the type of assistance the eidim require, or, more precisely, the exact hurdle they must overcome. Technical hurdles – such as kivan she-higid, which prevent them from testifying a second time - can be overcome through outside assistance. Kivan she-higid, according to the Ramban, does not challenge their dependability. Instead, it presents a technical and formal constraint against beginning a second testimony. This constraint may be overcome through the employment of migu. By contrast, eidim involved in a trei u'trei stalemate have actually testified but have seen that testimony opposed and neutralized by contravening eidim. They seek a boost through some outside force to overwhelm the competing eidim. Eidim who require outside forces to boost their 'ne'emanut' cannot be considered eidim.
That same Ramban allows migu to serve in an additional capacity, in enabling eidim to hurdle legal and formal obstacles which generally stifle their testimony. The gemara in Kiddushin (43b) describes eidim who were previously dispatched to return money from a borrower to a lender. Conceivably, their testimony about actually delivering the money to the lender is inadmissible since they have vested interest in such testimony. If we cannot establish the actual delivery of the money, they are vulnerable to prosecution from the borrower who will challenge them to explain their failure to properly perform their assigned task of delivery. Therefore, they might be considered a nogei'a bedavar (a prejudiced eid). The gemara, though, defends their ability to testify on the grounds that they have a migu: "Believe us when we say that we actually delivered the money to the lender, for if we wanted to lie, we could have claimed that we returned the money to the borrower" - a claim which would have been equally accepted. This seems to be an additional gemara endorsing the use of migu to assist eidim in surmounting the concern of nogei'a be-davar. The Rambam associates this gemara with Ketubot (18b). Here, too their eidut doesn't suffer lack of strength. Instead, there is a factor PREVENTING them from even beginning their testimony - the presumed vested interest. By supplying a migu, we actually liberate them from the nogei'a concern and allow their testimony to occur. Assistance at this stage is acceptable.
Tosafot in Kiddushin (43b) appear to disagree with the Ramban. They question the possibility of eidim taking an oath immediately prior to their testimony, in a manner which resolves the nogei'a concern and would allow them to commence their testimony. Tosafot disqualify this situation based on the aforementioned concern that eidim must be trusted in a self-sufficient manner without requiring outside assistance. As stated earlier, Tosafot provide an actual derasha from the pasuk which they believe asserts this condition. Even though the shevua would only address peripheral and technical concerns allowing the issuing of testimony, Tosafot do not allow any form of assistance. They are therefore, forced to explain the employment of migu to solve nogei'a (clearly assumed by the gemara in Kiddushin) in a different manner, which will not be addressed in this shiur.