Sotah, Chapter 6, Mishnah 3. In the previous Mishnah we learned the law that if a single witness testifies that at the time the woman secluded herself with the man about whom she had been warned she committed adultery, she does not drink the sotah waters. Our Mishnah comes to explain the basis for this law and its source in the verses.
The Kal Vachomer That One Might Have Made:
The Mishnah opens and says "shehayah badin" - that is, by strict logic there would have been room to construct a kal vachomer and conclude the opposite, and this is how it would have run:
"U'mah im edus rishonah she'ein osartah issur olam" - the first testimony is the testimony that she entered a secluded place with the man about whom she was warned. The very fact that she secluded herself with him is not so severe, since it does not forbid her forever: the prohibition that arises can be resolved by her drinking the sotah waters, being proven innocent, and returning to her husband as permitted. It emerges that the prohibition created by the seclusion is not permanent.
"Ein miskayem davar b'fachos mishnayim" - and nevertheless, this testimony is established only through two witnesses.
"Edus acharonah she'asasah issur olam - eino din shelo tiskayem b'fachos mishnayim?" - the later testimony is the testimony that she in fact committed adultery, and this testimony forbids her to her husband forever. If so, all the more so it should not be established with fewer than two witnesses.
This is the argument: if the lighter prohibition, which is not permanent, created by the seclusion alone, requires two witnesses, then surely the testimony about the act of adultery itself, which forbids her forever, should require two witnesses.
The Torah Teaches:
To counter this the Torah comes and teaches "V'ed ein bah" - that in the situation after she secluded herself with the man there is no witness against her. From here we learn "kol edus yesh bah": only when there is no testimony at all does she drink the sotah waters, but if there was any testimony at all against her, even a single witness alone, she does not drink the sotah waters.
The Reverse Kal Vachomer:
Now the Mishnah raises the argument in the opposite direction: "Vahalo kal vachomer l'edus harishonah me'atah" - once it has been established that a single witness suffices for the testimony about the adultery, let us construct a kal vachomer regarding the testimony about the seclusion:
"U'mah im edus acharonah she'osartah issur olam harei hi miskayemes b'ed echad" - the testimony that she committed adultery, which forbids her forever, is established even with a single witness.
"Edus harishonah she'ein osartah issur olam - eino din shetiskayem b'ed echad?" - the testimony about the seclusion, which does not forbid her forever and can be nullified by drinking the sotah waters, should all the more so be established with a single witness. Why, then, do we say that testimony of two witnesses is required that she was in a concealed place in order for her to drink the sotah waters?
The Gezeirah Shavah of "Davar" "Davar":
To counter this the Torah teaches "Ki matza bah ervas davar" - a matter of sexual impropriety, which is a hint to the seclusion with the man. And elsewhere in the Torah it says "Al pi shnayim yakum davar". From here we derive a gezeirah shavah: just as the "davar" mentioned there is established through two, so too the "ervas davar" of the seclusion is established through two, that is, through two witnesses.
The Conclusion of the Gemara:
In practice, the conclusion of the Gemara is that this gezeirah shavah is not needed for the testimony about the seclusion, but rather for a case in which there was neither a warning nor a seclusion, but only a general testimony that a woman committed adultery, where two witnesses are required. The requirement of two witnesses for the seclusion is derived from a different source: from the verse "V'ed ein bah", that there is no witness against her regarding the act of adultery itself. And the Gemara expounds: "bah" and not regarding the warning, "bah" and not regarding the seclusion - that only regarding the act of adultery itself does a single witness suffice, but for the warning, which is the formal caution, and for the seclusion, which is the secluding itself, two witnesses are required.
In summary: In this Mishnah we learned the source of the law that a single witness prevents the drinking of the sotah waters. The Mishnah rejected the kal vachomer that would have required two witnesses even for the testimony about the adultery, on the strength of the verse "V'ed ein bah" - "kol edus yesh bah", and it also rejected the reverse kal vachomer that would have been satisfied with a single witness for the testimony about the seclusion, on the strength of the gezeirah shavah of "davar" "davar". And we learned the conclusion of the Gemara: for the warning and for the seclusion two witnesses are required, while for the act of adultery itself a single witness suffices.