Bava Kamma 104

1:
Our mishnah presents two possible situations. In the first situation one person admits that he received money from the other person; however, he is not sure whether or not he returned the money. In the second situation he is not even certain that he ever received any money.

2:
The first clause of our mishnah is quite clear. David admits that he received 100 dinars as a loan from Joel. However, he thinks he may have returned the money, but he is not sure of it. In such circumstances David must pay Joel 100 dinars. He admits that he received the money and he can offer no proof that he ever made repayment. Joel, of course, is certain that no repayment was made. Joel is positive in his claim; David cannot be positive with any claim. Therefore, David must pay Joel 100 dinars. The legal maxim of the sages is "'certain' versus 'uncertain' – 'certain' prevails" [BK118a].

3:
In order to have a clearer understanding of the precise circumstances associated with the situation described by our mishnah the Gemara [BK118a] quotes a different teaching of the sages. In this teaching we learn that one person claims 100 dinars from another, but the other is not certain whether he did or did not receive the money. (It makes no difference whether we are concerned with a loan or theft or a deposit: in all such cases the first party is claiming that his property is in the possession of the second party and now demands its return.)

4:
To make it easier to understand let's introduce another scenario of our own. David confronts Sara and demands repayment of the 100 dinars that he deposited with her for safe keeping prior to his departure on a journey to Italy. Sara responds that she is not certain that she ever did receive the money from him.

5:
In the Gemara two sages hold that Sara would not have to pay David anything and two other sages hold that she would have to make payment. The reasoning of these two sages is the same as we have already encountered in the first clause of our mishnah: we have one statement of positive certainty against another statement of clear uncertainty. In such cases the rule would be that certainty must overrule uncertainty. The reasoning of the first two sages is that so long as David does not offer proof (such as a receipt or eye witnesses) his word is no more certain than Sara's. This, they would argue, brings us back to the general rule that we have encountered on more than one occasion, that "the burden of proof lies with the claimant": the money is in Sara's possession so David must prove that the money is, in fact, his.

6:
The Gemara now contrasts this with what is said in the second clause of the mishnah: "But if he says to him, 'I don't know if you loaned me the money' – he is not liable to pay."

The Gemara is here trying to rule out a certain possibility. Assuming that both clauses are referring to a situation in which the plaintiff made no demand of the defendant, why should there be no liability? He admits, under no kind of compulsion, that he did receive the money!

7:
The Gemara, therefore, says that the surmise that no demand was made must be erroneous and we must understand the mishnah differently: it must refer to a case where a demand for payment was made. But if this is the correct understanding of the mishnah how can the ruling be "he is not liable to pay"?

8:
This latter situation is a legal absurdity: if a demand for payment was made why should the defendant not be liable for payment at all? So the Gemara reverts to the original assumption: in the first clause no demand for payment had been made. The defendant, of his own free will, admits that he received the money: he is just not sure whether he had returned it or not. Why would a person admit such a thing? Because, says the Gemara, he wants to have a clear conscience.

The Gemara now bolsters this assumption with a teaching of Rabbi Yoḥanan: