Commentary for Bava Metzia 6:2

But [when dealing with] the testimony of witnesses, where this cannot be said, [perhaps there is no oath required]. We must remind ourselves that the Gemara is in the process of learning that when a defendant who claims that he owes nothing of the hundred zuz claimed by the plaintiff, is partially contradicted by witnesses who testify that he owes at least fifty zuz, he must swear about the other fifty, just as he would have to swear about the other fifty zuz, if he admitted that he owed fifty. We are saying that it is no difference between whether he is liable for the first fifty a) by his own admission or b) via the testimony of witnesses. In either case he must swear to defend his denial of the second fifty.

a)
Plaintiff: 100 zuz
Defendant: 50 zuz
Witnesses: N.A.
Ruling: Torah oath
b)
Plaintiff: 100 zuz
Defendant: Nothing
Witnesses: 50 zuz
Ruling: Torah oath

The Gemara has explained the psychology of requiring an oath when a) he admits fifty and denies fifty. The Gemara is now saying that the same psychology cannot be applied when he is liable for the first fifty zuz b) via witnesses. The Gemara uses an elusive phrase - the same cannot be said for when
a) the liability for the first fifty zuz was established by witnesses as for when
b) the first fifty zuz established by his own admission.
But what is truly the difference. Let us remember that in the previous Tosfos we learned that when a person denies half, it is not viewed as an attempt to steal. Rather, he is evading the creditor and stalling for time. This stands to reason when there was at least a partial admission of debt, but when there is a total denial, it is a bit more difficult to view the defendant’s denial as evading the creditor and stalling for time and not as an attempt to steal.
Tosfos says that this is what the Gemara means when it uses the phrase that when his liability of the first fifty zuz is established b) by witnesses, it cannot be said so readily,¹As we will soon see, Tosfos is not saying that it is impossible to view a total denial as an attempt to evade the creditor. He is merely saying that we cannot view him as evading the creditor so readily, as we can view a partial denial. that he is evading the creditor and stalling for time, since he denied the claim entirely. I should therefore say, since he is suspected of stealing, he is also suspected of swearing falsely.
The Baraisa of R’ Chiya informs us of the kal v’chomer which teaches us that [the defendant] is not a thief²See חידושי הר"ן who asks a very obvious question: If we have a legitimate point when we say that one who totally denies is viewed as attempting to steal and not as evading the creditor, how can the kal v’chomer teach us otherwise? Our logical argument cannot be refuted by a kal v’chomer. See חידושי הר"ן for a different explanation of the Gemara. Also see בית הלוי חלק ג' לז ו' for an answer to this question according to Rashi’s explanation of the Gemara. [by denying liability] because he too is evading the creditor and stalling for time, but his intention is not to steal.
Tosfos proves that even a defendant who denies everything is not considered a thief: For one who falsely denies owing a loan is qualified to be a witness. If he was suspected of thievery, we could not accept his testimony in bais din. Since his testimony is acceped, it is evident that we do not think of him as a thief.
Was it necessary for the Tanna to teach us that we do not say that since one is suspected of stealing, he is also suspected of swearing falsely? Even though [the Gemara] later (6a) says that we do not say ‘since he is suspected etc. [of stealing, he is also suspected of swearing falsely]’. If so, why does the Baraisa of R’ Chiya need to teach us this, there is already adequate proof of this concept?
Even so, the Tanna informs us that we do not say: Since he is suspected of stealing, he is also suspected of swearing falsely. This he derives from a kal v’chomer from two sources, the oath that he must take when he admits partial liability with his own mouth, and the oath that he must take when a single witness testifies against him.
The Gemara on 4a is attempting to understand the kal v’chomer that teaches us that b) when witnesses testify that the defendant owes at least fifty of the hundred zuz claimed by the plaintiff, the defendant must swear about the fifty zuz that the witnesses are not testifying about.
The Gemara has shown that if the source of the kal v’chomer is the oath of partial admission or the oath of a single witness, the kal v’chomer is not functional. This process if known as a ‘פירכא - challenge’ to the logic of the kal v’chomer. The kal v’chomer is constructed on the logic that if “A” which is lenient requires an oath, then “B” which is more stringent certainly requires an oath. The ‘פירכא - challenge’ shows that in a certain way “A” is actually more stringent, thus undoing the basis of the kal v’chomer.
The Gemara on 4a summarizes the information discussed about the kal v’chomer: One’s own mouth is proof a) that when we know that he is liable for half the claim, that he must swear about the other half. So too, b) when his partial liability is established by the testimony of witnesses, he must swear about the other fifty zuz. What can you prove from the oath that one must take by a) the admission of his own mouth which cannot be contradicted. An admission is extremely powerful. Even if witnesses contradict an admission, the defendant remains liable. If so, we cannot derive that b) when witnesses testify that the defendant owes half, he should be obligated to swear from the oath that he must take when he admits owing half, because his own admission that he owes half is more potent than the testimony of witnesses that he owes half. Perhaps it is because of this potency that the Torah requires an oath when one admits half, but not when it is the testimony of witnesses that causes him to pay half.
The oath obligated by a single witness will prove that even if the testimony can be contradicted it can obligate an oath, for the testimony of a single witness can be contradicted and even so, it obligates the defendant to swear.
What can you prove from the oath obligated by a single witness, where the oath will be for the money that the witness testified about? He denied owing any money and the witness testifies that he owes a hundred zuz. He must swear to contradict the witness' testimony which supports the claim of the plaintiff. Can we derive from this that when two witnesses testify that the defendant owe fifty zuz, the defendant is obligated to swear about the other fifty zuz, which he denied owing - a claim which is not supported by the testimony of witnesses?
The oath he is obligated to take when he admits owing half by his own mouth, proves that one can be liable to swear about the second fifty zuz, even though the claim of the plaintiff is is not supported by the testimony of any witness.
At this point we can prove from
A) the oath obligated by a single witness, that one can be obligated to swear by testimony that can be contradicted. We can prove from the oath obligated by one’s admission that one can be required to swear
B) even on the portion of a claim that is unsubstantiated by either one’s own admission or by a single witness.
From the combination of A) and B), we can prove that b) when witnesses testify that the defendant owes half the money claimed by the plaintiff, he is required to swear about the other half.
Tosfos refers to learning from this combined source as deriving this ruling from the oath obligated by פיו - the admission of his own mouth and from the oath obligated by an עד אחד - the testimony of a single witness.
What emerges from our Gemara is that there are two reasons why we might not be able to derive that b) when partial liability is established by witnesses an oath is required, from when partial liability is established by his own admission:
A) When he admits partial liability, we view the partial denial as an attempt to evade the creditor and as a method of stalling for time. Whereas, when one has already totally denied liability, it seems as if he is attempting to steal. The Gemara uses this reason to prove that it is necessary to have a kal v’chomer, otherwise we could not compare the testimony of witnesses to his own admission.
B) One’s a) own admission in a sense is stronger than b) witness’ testimony. His admission cannot be contradicted by witnesses, nor can they invalidate his admission by saying that he was not in the place where the supposed loan took place. For example: He admits that he borrowed money from the plaintiff in New York City on January 1st. Witnesses say that they were with the defendant on January 1st in Los Angeles. The loan that the defendant admits to could never have happened according to the witnesses. Even so the admission stands. The Gemara uses this reason as a challenge to the kal v’chomer.
The Gemara initially said that there is an inherent difference between a) when one’s partial guilt is established by admission and b) when it is established by witnesses.
The Gemara says that we cannot derive liability b) when the admission is established by witnesses from a) when it is established by one’s own admission, because of reason A), when partial liability is established by witnesses, he has already denied owing money and is viewed as having attempted to steal.
It seems that the Gemara could have used reason B), just as easily. We would think that partial liability established by witnesses does not obligate an oath because one’s admission is stronger than the testimony of witnesses, since it cannot be contradicted or invalidated. So too, when the Gemara suggests reason B) as a challenge to the kal v’chomer, it could just as easily have used reason A).Tosfos would like to know why the Gemara initially uses reason A) to show that there is a need for the kal v’chomer and later switches to reason B) to challenge the kal v’chomer.
If you ask: Why does [the Gemara] need reason A) that a) when partial liability is known from the defendant’s admission, he is not viewed as a thief because we say that he is evading the creditor and stalling for time, as opposed to b) when his partial liability is established by witnesses, where we view him as having attempted to steal. Let [the Gemara] say that if not for the kal v’chomer, I would think reason B), that one’s own admission is more potent than the testimony of witnesses, because it is not subject to contradiction or invalidation? Therefore, one’s own admission of partial liability can obligate an oath, but partial liability by witness’ testimony cannot obligate an oath.
There is a similar problem with the Gemara that presents the challenge to the kal v’chomer. So too, later, [the Gemara] should have challenged the kal v’chomer from a) one’s admission by arguing - what can you prove from one’s admission where the defendant is viewed as evading the creditor and stalling for time, to b) when we know of his partial liability by witnesses, but he himself has totally denied liability and cannot be viewed as evading the creditor?
We can answer: That here, where [the Gemara] is explaining the meaning of the phrase we should not think “that his own admission is greater and more likely to require an oath, than the testimony of witnesses”, [the Gemara] is more comfortable using reason A): that [the debtor] is admitting to only half because he wants to evade the creditor and stall for time, for it is logical that for this reason, one would be required to swear a) when his partial liability is established by his own admission, but not b) when his partial liability is established by the testimony of witnesses.
However, [the Gemara] is more comfortable citing the potency of one’s admission, that it cannot be contradicted or invalidated, later to annul the kal v’chomer, for that is the method of the Gemara, to raise a challenge against a kal v’chomer, which shows that what is presented as ‘kal- lenient’ such as one’s own admission, is not lenient at all because it cannot be contradicted or invalidated. But [the Gemara] does not use logical arguments to reverse a kal v’chomer, therefore, it does not cite the logical difference between a) one’s admission and b) the testimony of witnesses, that in the case of one’s own admission his partial denial can be seen as an attempt to evade and stall for time from the creditor.