Machshirin, Chapter 3, Mishnah 2. This chapter continues to work out when produce that has become wet counts as having been wet beratzon, with the owner's approval, so that it becomes hechsher lekabel tumah, ready to receive tumah. Our mishnah takes the case of liquid that never was poured onto the produce at all: it seeped through the wall of a barrel.
Two situations are laid out here. The first: "Chavis shehi mele'ah peiros unesunah lesoch hamashkin" - a barrel loaded with produce that has been placed down into liquid. The second is the reverse arrangement, a barrel that is "mele'ah mashkin" (filled with liquid) and which is then "unesunah lesoch peiros" (set into produce). In both pictures the mishnah concludes, "veshav'u, kol sheshav'u bechi yutan": since the liquid soaked its way through the side of the barrel, the produce it dampened is regarded exactly as if a person had wet it on purpose, and it has now acquired hechsher lekabel tumah.
The Rav points out that we are necessarily dealing with a barrel whose material lets moisture pass, unglazed clay, for instance, or a variety of stone that absorbs. Only a wall of that sort permits liquid to travel across it. When the produce is packed inside and the liquid surrounds the barrel from without, the dampness works its way inward; when the liquid is within and the produce is piled outside, the dampness travels outward and reaches the produce pressed up against the barrel. In each direction the result is the same: produce moistened by a barrel that is standing flush against it and in contact with it has been wet bechi yutan.
Which liquids seep through
The mishnah then narrows the ruling: "Ve'eilu mashkin amru: bemayim, uveyayin, uvechometz, ushe'ar kol hamashkin tehorin." Of the seven liquids that are able to bring about hechsher lekabel tumah, only three are involved here: water, wine and vinegar. These three are thin enough to permeate the porous wall of the vessel and reach the produce on the other side. The remaining liquids do not pass through, and produce lying alongside a barrel filled with them stays tahor.
The obvious difficulty is this. Granted that in the ordinary course of events the other liquids stay inside the barrel; but suppose in some case one of them did in fact seep through and wet the produce. Why should that not create hechsher lekabel tumah? The answer lies in the definition of bechi yutan. As we have learned, for wetting to count it must be something a person would be pleased to have happen. Since these liquids are not expected to pass through the wall of a vessel, even when it happens the result is not bechi yutan, because this is simply not the normal course of things and no one anticipated it. Water, wine and vinegar, by contrast, are expected to seep through, so the wetting they cause is regarded as beratzon and therefore as bechi yutan.
The view of Nechemyah
The mishnah closes with a dissenting position: "Nechemyah metaher bekitnis, she'ein hakitnis sho'eves." Nechemyah holds that legumes, beans, peas and the like, remain tahor in these cases. His reason is that such produce does not really draw liquid into itself. Even when the surface becomes damp, nothing is absorbed, and a wetting of that sort is not something a person cares about, so it is not beratzon and not bechi yutan. This, however, is not the accepted halachah; we do not pasken according to Nechemyah.
The mishnah thus teaches a fine but consistent principle: the measure of bechi yutan is not merely whether liquid arrived, but whether its arrival was the expected and welcome result of the way the owner arranged his produce.