In one of the Commentaries (…) we read of an interesting case. A certain man, wanting to marry the daughter of a rich merchant, said to him (since the daughter was under-age): “Behold, your daughter is betrothed [mekudeshet] to me” There was one catch: the merchant actually had three daughters, and the man had neglected to mention which one he meant. (The halachic issue is that if he does not want to marry the girl, he must give her a “get”, so that she can marry someone else.)
Rabbeinu Tam, one of the great decisors of Jewish law (and the grandson of Rashi), ruled that in the absence of a specific statement by the man as to which daughter he intended, the assumption was that the eldest daughter was intended. (Other commentators argued with this, but that does not concern us here.)
And that is how we decide until today — if a man wants to marry a woman who has an unmarried elder sister, she must first get permission from that sister.
The interesting thing is how R’ Tam came to this decision: he quoted the statement in this week’s parsha by Lavan, when taken to task by Jacob for giving him the “wrong” daughter, Leah, instead of Rachel, in marriage: “It is not so done in our place, to give the younger before the first-born” (Gen. 29:26).
We may ask how R’ Tam could learn halacha from a “rasha” (wicked man) like Lavan — especially since Jacob, a righteous man, argued with him, and he had got his way by trickery!
There are other examples of halachic decisions apparently made by wicked men.
For example, the Shulchan Aruch ( … ) rules that in kashering glassware (although this is less of an issue than kashering other implements), the glassware should nevertheless be immersed in water for three days, with the water changed each day.
From where is this law derived? The commentators ask this, and one of them found the source — in a Midrash on Megillat Esther, Haman said to King Achashverosh, in order to antagonize him against the Jews: “Do you know that if a fly falls in a Jew’s wine cup, he simple pulls the fly out and goes on drinking the wine, but if you, O king, touch that wine, he must throw it out, and wash the cup three times!”
And we follow the Shulchan Aruch’s ruling, obtained apparently from a wicked man who wanted to destroy the Jews!
Here is a third example:
There are laws of privacy forbidding one’s house having a window through which one can see into a neighbor’s window. The source of this is Bilaam, who tried to curse the Jews, and instead found himself blessing them, with the words “How goodly are your tents, Jacob, and your dwelling-places, Israel!” (Rashi on Num. 24:5).
I once collected 24 such examples of Jewish law apparently being derived from wicked men.
How can we explain this?
The answer is, I think, this. If someone has a failing, he becomes tainted in our eyes; we no longer trust him in anything. But the Torah’s approach is that we should judge everyone’s statement on its own terms, and not simply reject it because of our opinion of the speaker.
The Sages decided that Lavan, however deceitful he was, and whatever his motives, had a point — we should be concerned about the feelings of an unmarried elder sister.
Similarly with Haman — the Rabbis must have decided that in kashering glass, it might not be such a bad idea to show a little care.
As for Bilaam, the Rabbis must similarly have felt that some concern for privacy and modesty could be learned profitably from his statement.
From the Torah’s perspective, no-one, however wicked, is completely worthless in what he has to say. Everyone has his moment of truth.