2,698. Oaths of Testimony vs. Oaths of Expression

Hilchos Shevuos 9:18

Let’s say that person Y (a witness) makes an oath to person Z (a defendant) to the effect that he has information relevant to his case. It later turns out that person Y did not, in fact, have any information. In such a case, person Y is not liable – neither for an oath of testimony nor for an oath of expression. This is because an oath of expression only applies to things that have both a positive and a negative aspect. If person Y had made an oath that he didn’t have information, that wouldn’t be an oath of expression, it would have been an oath of testimony. Since the negative aspect isn’t an oath of expression, the positive aspect – i.e., an oath that one does have information – can’t be an oath of expression.

Hilchos Shevuos 9:19

It should go without saying that if someone takes an oath that he testified when he really didn’t, or that he didn’t testify when he really did, then he’s liable for an oath of expression. This has nothing at all to do with an oath of testimony.