palginan diburei and teaching lomdus

My son is learning Makkos this year and and I have noticed that he grasps gemara with rashi well from what he learns in school, but moving him beyond that to conceptual thinking (i.e. lomdus) is a bit of a struggle. I’m not sure if grasping more abstract analysis is something one picks up with maturity (there is a reason algebra is not taught before high school) or is it a matter of being exposed to enough of it until one assimilates the thought patterns. The nafka minah is whether it pays to provide exposure to lomdus in the hopes that it rubs off, or take a wait and see attitude. I try to provide some exposure, with little results so far, but I’m patient... (those who know me will undoubtedly recognize that the last half of that sentence was a joke : )

Last night we were reviewing the gemara (7a) which raises the question of whether eidim who are related to a cosigner (areiv) of a loan are disqualified from testifying about the loan. The gemara concludes that they are disqualified because of their relationship being that the areiv must pay if the debtor defaults.

The Rosh here raises a famous question. Elsewhere the gemara introduces the concept of palginan diburei, splitting testimony. If Reuvain testifies that Ploni is pasul l’eidus because he charges interest on loans, and Shimon testifies that Ploni is pasul l’eidus because he charged him interest on a loan, we accept their testimony and believe Ploni charges people interest. How can we believe Shimon when his testimony implicates himself and ain adam meisim atzmo rasha? The answer is that Shimon’s testimony really consists of two parts: 1) Ploni lends people money with interest; 2) Shimon himself was lent money and charged interest. Palginan dibiuei allows us to split testimony; we reject the part which contains Shimon’s self-implication but accept the rest.

What is the difference between a case of palginan diburei and the case of the relatives of the areiv who want to testify? In effect, their testimony consists of two parts: 1) the debtor took a loan which must be repaid; 2) the areiv agreed to repay if the debtor defaults. Why not say palginan diburei and accept their testimony with respect to the debtor’s obligation, and reject the portion of their testimony that relates to the areiv (to whom they are related)?

I like to throw these type issues at my son to try to force him to be medameh milsa l’milsa, which is really what all advanced learning boils down to. I don’t want to spoil the fun of thinking about it if you haven’t seen this Rosh, so I'll leave off without posting an answer.