Tevul Yom, Chapter 4, Mishnah 1. Before this mishnah can be understood, one basic rule of tumah has to be in place: dry food does not receive tumah at all. Produce becomes susceptible to tumah, huchshar lekabel tumah, only once it has come into contact with one of the seven liquids that serve as a machshir. A loaf that was baked from wheat which never met any of those liquids is simply outside the system; nothing can render it tamei. The Mishnah now asks what happens when the liquid that reaches such a loaf is a person's own saliva.
The opening case features someone who is completely tahor. The mishnah describes him as "tahor shenagas min ha'ochel", a tahor person chewing on his food, and while he chews some saliva escapes and settles first on his garment, "venafal al begadav", and travels from there to a loaf of terumah, "ve'al kikar shel terumah". The loaf, remember, had never been rendered susceptible to tumah by any of the seven liquids. Has this saliva now done the job? The ruling is "tahor". The saliva carries no halachic weight as a liquid, because it was never wanted: he had no use for saliva pooling in his mouth, and he obviously had no interest in its reaching the loaf. Unwanted liquid does not serve as a machshir.
Now the menu changes. "Hayah ochel zeisim petzu'im utemarim retuvos": his meal consists of olives that have been split open or dates that are still soft and juicy, and the rule extends to "kol shehu rotzeh limotz es gar'inoso", any fruit whose pit he plans to suck on once the flesh is gone. Here the verdict flips to "tamei", that is, the loaf has now become susceptible to tumah. What changed? A person who means to work on the pit does want saliva accumulating in his mouth. Granted, its landing on the terumah loaf was no part of his plan, but wanting it in his mouth is already enough to give it the halachic standing of a drink, and a liquid with that standing acts as a machshir at whatever it touches.
The third case brings us back around. "Hayah ochel zeisim neguvim utemarim yeveishos", his food is olives that have dried out and dates that hold no moisture, and the same applies to "kol she'eino rotzeh", whatever fruit he has no desire "limotz es gar'inoso", to draw on its pit afterward. Should saliva escape as he eats, land on his garment, and pass from there onto a loaf of terumah, we return to the ruling of "tahor". Since the saliva was never wanted in his mouth to begin with, it never attained the halachic standing of a liquid, and it leaves the loaf as unsusceptible to tumah as it was before.
The Mishnah now brings a tevul yom into the discussion. A tevul yom is someone who had been tamei, immersed in a mikveh, and is waiting for the day to end before he is fully tahor. He stands in an intermediate position: the immersion has already taken effect, yet his purification is not complete until nightfall. The Tanna Kamma teaches "echad tahor ve'echad tevul yom", all of the above rulings hold equally whether the one eating is fully tahor or is a tevul yom. The status of his saliva is decided purely by whether he wanted it in his mouth.
"Rabbi Meir omer: eilu va'eilu temei'in bitvul yom." Rabbi Meir disagrees. In his view, even the cases the Tanna Kamma declared tahor, where the saliva was unwanted, come out tamei when the one eating is a tevul yom. His reason: "shemashkin shel tamei machshirin lirtzono veshelo lirtzono", the liquids of a person who is tamei make food susceptible to tumah whether he wanted them or not. For a tamei person, desire is not part of the equation, and Rabbi Meir treats the tevul yom accordingly.
"Vachachamim omrim: ein tevul yom tamei." The Chachamim answer that the principle Rabbi Meir invokes is correct in itself, but it belongs to a tamei person, and a tevul yom is not one. He has already immersed and is well along the road to taharah; for these purposes he is essentially tahor, lacking only the arrival of nightfall. His saliva is therefore judged exactly like the saliva of a fully tahor person, and it makes food susceptible to tumah only when he wanted it in his mouth. That is the machlokes with which this mishnah concludes.