Taharos, Chapter 4, Mishnah 6. This chapter is devoted to cases of safek, doubtful tumah, and to the question of when we act stringently and when we do not. The preceding mishnah listed six sefeikos regarding which the Chachamim issued a decree of stringency, going so far as to have terumah burned on their account. Our mishnah now takes one of those cases, the spit of a zav found in a public place, and works out its details step by step.
The mishnah begins: "Shnei rukin, echad tamei ve'echad tahor", two globs of spit, one of them impure and one of them pure. Notice what is certain here and what is not. It is entirely certain that one of these two is the spit of a zav, which conveys tumah, and it is entirely certain that the other one is perfectly clean. What we do not know is which of the two we came into contact with.
A zav's spit transmits tumah through three distinct channels: maga, direct contact with it; masa, bearing its weight even without touching it; and heset, causing it to move or be jostled. The mishnah addresses each of these three avenues, teaching that "birshus hayachid", in a private domain, where doubtful tumah is ordinarily judged strictly, terumah that this person handled is neither eaten nor destroyed. Its status is left unresolved, because the question of which of the two globs was involved has no answer.
Now the mishnah narrows the branches of the case. In a public domain, where doubtful tumah is generally treated leniently, the outcome depends on the condition of the spit: "al maga'an birshus harabim bizman shehen lachin". As far as touching is concerned, we suspend the terumah in a public domain only when the spit is still moist. "Ve'al masa'an bein lachin uvein yeveishin", but as far as carrying goes, we suspend the terumah whether the spit is moist or dry.
Why should moisture make a difference? Moist spit clings to whatever it meets. Once it adheres to a person or to an object, it has, in effect, formed a private domain of its own around itself, and the leniency of the public domain no longer applies in the same way. And a word of clarification about the word "dry": we are never speaking of something entirely dried out, because spit that has fully dried is no longer spit at all and conveys nothing. The distinction is between the moister and the drier state of the same substance.
The mishnah closes with a different situation altogether: "hayah rok yechidi venaga bo, unsa'o, vehesito birshus harabim, sorfin alav es haterumah, ve'ein tzarich lomar birshus hayachid". Set aside the case of two globs. Here there is only one glob of spit, and a person touched it, carried it and moved it, and all of this took place in a public domain, the very setting in which we are ordinarily lenient. Even so, terumah that he then touched is burned, and needless to say the same holds in a private domain.
The logic follows from what the previous mishnah taught us. The spit of a zav in a public place is precisely one of the cases in which the Chachamim decreed stringency. In the earlier part of our mishnah there was an additional layer of uncertainty piled on top of that, since we did not know which of the two globs we had interacted with, and that extra doubt was enough to leave the terumah suspended rather than burned. Here that second doubt falls away. There is one glob of spit and we know exactly which object we handled. With only the single doubt remaining, the decree of the Chachamim applies in full force, and the terumah is burned.