The arrangement of wine presses during Talmudic times was such that there were actually two separate areas hewn out of the rock, one above the other, with a connection between them. In this way the grapes could be placed in the upper press where they were trampled, allowing the grape juice to flow into the lower area.
The Gemara on today’s daf (=page) refers to this wine press arrangement in the context of a question about terumah wine that becomes mixed with ordinary wine that has been ritually defiled. When ordinary wine is tameh (ritually defiled) it can be drunk by anyone who is, themselves, in a state of ritual impurity, or, indeed, by anyone who was not careful to keep the stringency of drinking only wine in a purified state. Terumah wine, which is separated as one of the priestly offerings, has a high level of sanctity, can only be drunk by kohanim and cannot be drunk by anyone if it has become tameh.
According to the Mishnah in Masechet Terumot (8:9), in a case where a barrel of terumah wine breaks and spills into the upper chamber of the wine press described above, and the bottom chamber contains ordinary wine that is tameh, we face a challenge. Assuming that there is not enough wine in the lower chamber to nullify the terumah wine spilling in, once theterumah wine mixes with the ordinary, tameh wine, none of it can be drunk, not even by kohanim. The Mishnah teaches that if even a small amount – a revi’it – can be saved by finding pure vessels and drawing out wine from the upper chamber, that is what should be done. If no pure vessels can be found, Rabbi Eliezer rules that we cannot collect theterumah wine in vessels that are tameh in the interest of keeping it from spilling into the lower chamber (which would render all of the wine unfit for consumption). This is because Rabbi Eliezer does not allow anyone to actively defileterumah. Rabbi Yehoshua disagrees, arguing that since in any case this terumah wine will become defiled when it falls into the lower chamber, it is permissible to collect it in tameh vessels in the interest of saving the ordinary wine in the lower chamber.