Sotah, Chapter Six, Mishnah Three
This mishnah provides the midrashic basis for why two witnesses are necessary to testify that the sotah was secluded with the suspected man but one witness is sufficient to testify that she had been defiled (committed adultery). The mishnah notes that this is counterintuitive, for the second type of testimony is surely more consequential. Therefore, it proves this halakhah using strict exegetical methods, methods which are allowed to be counterintuitive.
1) For it would have been logical: Now if the first evidence [that the woman had secluded herself with the man], which does not prohibit her [to her husband] for all time, is not established by fewer than two witnesses, is it not logical that the final evidence [that she had been defiled] which does prohibit her to him for all time, should not be established by fewer than two witnesses!
2) Scripture states, And there is no witness against her (Numbers 5:13)whatever testimony there may be against her [is believed].
3) And now with respect to the first evidence [about her seclusion with the man] there is an a fortiori (kal vehomer) argument: Now if the final evidence [regarding her being defiled], which prohibits her to her husband for all time, is established by one witness, is it not logical that the first evidence, which does not prohibit her to him for all time, should be established by one witness!
4) Scripture states, Because he has found some unseemly matter in her (Deuteronomy 24:1), and elsewhere [Scripture] states, By the mouth of two witnesses … shall the matter be established (Deuteronomy 19:15); just as the matter mentioned in this latter case must be based on the testimony of two witnesses, so also here [in the case of the suspected woman] the matter must be based on the testimony of two witnesses.
Section one: In order to understand this mishnah in its totality we need to understand that its intent is to defend the following halakhah: two witnesses are required in order to make the suspected adulteress drink the sotah waters. These two witnesses must testify that they saw her secluded with the man with whom her husband warned her not to be secluded. If two valid witnesses testify that she is secluded with him, her husband brings her to the Temple to drink the sotah waters, which will determine her guilt or innocence. If she is found innocent, she may return to normal married life. In other words, the result of their testimony may be temporary. However, only one witness is needed to testify that she actually committed adultery, as we learned in yesterdays mishnah. Seemingly this testimony should require two witnesses for its results are permanent. After testimony that she was defiled by committing adultery, she may never return to her husband. This is the logical problem brought up in section one of the mishnah.
Section two: The resolution is that this rule is learned from a counterintuitive midrash. Numbers 5:13 states, And there is no witness against her, using the singular of the word witness. Had there been a witness, even one witness, even a witness who is normally invalid (such as a slave), she does not go on to drink the water.
Section three: Now that the mishnah has established that one witness who saw her defiled is sufficient to cause her to be permanently prohibited to her husband, the requirement for two witnesses to testify that she was secluded does not make logical sense. Why should testimony regarding seclusion, whose consequences can be temporary, require two witnesses, while testimony regarding defilement requires only one?
Section four: Again, the answer is a midrash. Deuteronomy 24:1 states that if a man finds in his wife an unseemly matter, he may divorce her. The midrash assumes that this unseemly matter refers to her having been secluded with the suspected other man. The word matter is connected with the same word used in Deuteronomy 19:15, which states that two witnesses are required in order for any matter to stand. The linguistic connection means that in the case of seclusion, two witnesses are required.