Machshirin, Chapter 2, Mishnah 1. This chapter continues the tractate's central subject: which liquids are able to make food susceptible to tumah, and which are not. Our mishnah takes up a whole family of cases involving perspiration, both the moisture that collects on walls and in underground spaces, and the sweat that comes out of a person's body.
Before we begin, an important note about the vocabulary. When the mishnah here calls something tahor, it does not mean tahor as opposed to tamei in the ordinary sense. As the Rav explains, it means that this liquid is not machshir le'kabbel tumah: if food gets wet from it, that food does not thereby become susceptible to receiving tumah. And when the mishnah says tamei, it means the reverse: this liquid does count as a hechsher, so that food wet by it can now become tamei.
Condensation in dwellings and in underground spaces
The mishnah's first ruling concerns ze'as batim, along with the same phenomenon in boros, shichin, u'me'aros, and the verdict in every one of these cases is tehorah. What is under discussion is the film of water that gathers on the inner surface of a dwelling, or along the sides of a pit, a trench dug for water, or a cave. Such moisture carries no power of hechsher whatsoever. Should a piece of food be dampened by it, nothing has changed in that food's status, and it remains incapable of receiving tumah.
Human perspiration
"Ze'as adam tehorah." The sweat of a person is likewise tahor. Certain liquids that leave the human body do serve as a hechsher le'kabbel tumah, but sweat is not among them. So if food is moistened by the perspiration of a person's body, that food has not been rendered susceptible to tumah.
He drank tamei water and then perspired
The mishnah's next case begins "Shasa mayim teme'im": a person swallowed water that carried tumah, and the drinking brought on perspiration, concerning which the mishnah rules "v'ze'ah, ze'aso tehorah." There is room to reason the other way. Why not attribute the sweat to its source, the tamei water that entered him, and treat it accordingly? The mishnah rejects that line of thinking. What emerges from him is defined by what it now is, not by what it once was, and what it now is, is sweat. Since perspiration is not on the list of liquids that create susceptibility, food dampened by it has received no hechsher at all.
He entered drawn water and then perspired
The following clause reverses the outcome. "Ba b'mayim she'uvim", he stepped into drawn water, and then "v'ze'ah", he perspired: the ruling is "ze'aso teme'ah." Note the precise term mayim she'uvim, water that has already been separated from a source joined to the earth. That precision is not incidental. Water that remains connected to the ground has no capacity to be machshir under any circumstances, neither where the wetting was desired nor where it was not. It is exclusively water that has been detached and drawn that carries the power to create susceptibility.
So a person went into such drawn water, and immediately afterward he perspires. That moisture is tamei, that is, it does act as a hechsher. The reason is that the wetness now on his body is not regarded as having emerged from his sweat glands at all; it is treated as the drawn water itself, still clinging to him. If that liquid passes from his body onto food, the food has been rendered susceptible to tumah.
He dried himself first
Toweling off before the sweating begins
The closing clause reads "Nistapvag", he wiped himself dry, "v'achar kach ze'ah", and the perspiration came only afterward. In that situation the mishnah rules "ze'aso tehorah": what appears on him grants no hechsher, and food dampened by it stays outside the reach of tumah.
The line drawn by the mishnah is now clear. Only moisture that comes directly from the mayim she'uvim can serve as a hechsher le'kabbel tumah. Once he has dried off, whatever appears on his skin afterward genuinely comes from his own sweat glands, and sweat, as the mishnah taught at the outset, never renders food susceptible to tumah.