Parah, Chapter 11, Mishnah 4. This chapter travels back and forth between the halachos of mei chatas and the various levels of tumah and taharah connected with them. Here the focus shifts from the water to a human being: a person who has become tamei to a degree that the Torah itself requires him to immerse. Two questions are asked about him. What does he make tamei while he is still unimmersed, and what remains of his tumah once he has gone into the mikveh but the day has not yet ended? On the second question Rabbi Meir and the Chachamim part ways.
The Text
"Kol hata'un", every person who is obligated, "bi'as mayim", to come into water, meaning to immerse in a mikveh, "midivrei Torah", by force of Torah law and not merely by rabbinic decree, "metamei es hakodesh, ve'es haterumah, ve'es hachullin, ve'es hama'aser": such a person transmits tumah to kodesh, to terumah, to chullin and to ma'aser, "ve'assur bevi'as hamikdash", and he may not come into the Mikdash.
"Le'achar bi'aso", once he has gone into the water, while the sun has still not gone down, "metamei es hakodesh, uposeil es haterumah": kodesh he still makes tamei, while terumah he only disqualifies without making it tamei. "Divrei Rabbi Meir", so rules Rabbi Meir. "VeChachamim omrim: poseil bakodesh uveterumah, umuttar bechullin uvema'aser": according to the Chachamim he merely disqualifies in both kodesh and terumah, while with chullin and ma'aser there is no restriction on him at all.
"Ve'im ba el hamikdash", and should he come into the Mikdash, "bein lifnei bi'aso", either before his immersion, "bein le'achar bi'aso", or afterwards, "chayav", he bears liability.
While He Is Still Unimmersed
Our subject is a man who came into contact with an av hatumah, a primary source of tumah. That is why his immersion is a Torah requirement rather than a rabbinic one. Such contact places him in the category of a rishon, a first level of tumah.
Now, food touched by a rishon becomes a sheni, a second level. Hence when this man handles chullin, or ma'aser (that is, ma'aser sheni, since ma'aser rishon falls together with plain chullin for this purpose), he leaves them with second-level tumah. Food of that level can then give a third level to terumah, and terumah holding a third level can give a fourth level to kodesh. The mishnah's four-item list is really a shorthand for this descending ladder. Needless to say, before immersing he has no entry into the Mikdash.
Immersed, but the Sun Has Not Set
Going into the mikveh does not complete his purification. He is a tevul yom until sunset, and what is left of his tumah is the level of a sheni rather than that of a rishon.
Rabbi Meir draws out what follows. Being a sheni, he leaves a third level on anything he touches. Terumah in his hands becomes a shelishi, and terumah at the third level is pasul, invalid, but not tamei: it is ruled out for use, yet it has nothing further to transmit. With kodesh Rabbi Meir rules otherwise: here the tevul yom actually imparts tumah, so that this kodesh is able to give a fourth level to other kodesh.
For the Chachamim there is no difference between kodesh and terumah. Terumah touched by a tevul yom is invalidated and can transmit nothing onward; kodesh is no different, becoming a shelishi that is ruled out for use and unable to pass a fourth level to anything else. In their view his effect in both is disqualification alone.
All opinions concur about one detail: a tevul yom, who has already immersed and is simply awaiting sunset, may partake of chullin and of ma'aser sheni, and touching them raises no difficulty.
Coming into the Mikdash
The Mikdash stands in a class by itself, and immersion gains him nothing here. He carries the same liability for entering whether he has not yet immersed or has immersed and is waiting out the day.
A fine point deserves mention. This liability applies to the men's section of the Mikdash. He is certainly not free to walk into the women's court in his present condition, yet doing so does not carry that same liability. It is setting foot in the men's area that makes him chayav.