In the previous Mishnah we learned the statements of Rabbi Chanina, the deputy Kohen Gadol, and of Rabbi Akiva. Rabbi Chanina, the deputy Kohen Gadol, taught that sacrificial meat which is third-degree tumah may be burned together with sacrificial meat which is first-degree tumah, even though this raises the level of tumah of the third-degree meat and makes it second-degree. Rabbi Akiva expanded on this and ruled that with sacred oil one may even raise it two levels, from third-degree tumah all the way to first-degree tumah.
Rabbi Meir's ruling:
From these two statements the law of our Mishnah is derived, and this is also the reason the ruling of the previous Mishnah was brought here, because it bears on Erev Pesach. Rabbi Meir said: "Midivreihem lamadnu sheh'sorfin Terumah tehorah im hatemei'ah baPesach" - from the words of Rabbi Chanina, the deputy Kohen Gadol, and from the words of Rabbi Akiva we learned this law.
Terumah, which is given to the Kohen, may be eaten only while it is pure, and not once it has become tamei. Rabbi Meir rules that on Erev Pesach, at the time when the chametz is burned, one may burn pure Terumah, which is still fit to be eaten, together with Terumah that is tamei, even though in the course of this the pure Terumah will become tamei.
Rabbi Meir's reasoning: just as we were not concerned about raising the level of tumah of the sacrificial meat, since in any case it is destined to be burned and the matter makes no difference to us, so too here. Since the pure Terumah is about to be burned on account of the chametz on Pesach, it no longer makes any difference that it will become tamei.
Rabbi Yosei's objection:
Rabbi Yosei disagreed with him and said: "Einah hi hamidah" - the measures are not equal and the cases are not comparable to one another. In the source of the derivation, the sacrificial meat, we are dealing with something that was tamei from the outset, whereas here the Terumah is completely pure. It is possible that the leniency of burning the sacrificial meat together with something whose level of tumah is more severe was granted precisely because it already carried some degree of tumah - not the same degree, to be sure, but it was tamei. Not so with Terumah: even though it must be burned on account of the chametz, as far as the laws of Terumah and purity are concerned it carries no tumah at all, and therefore it may be that even on Erev Pesach it should not be burned together with Terumah that is tamei.
The dispute between Rabbi Eliezer and Rabbi Yehoshua:
Rabbi Yosei continues along his line of reasoning and brings the dispute between Rabbi Eliezer and Rabbi Yehoshua, citing it mainly for the point on which both of them agree with his position: pure Terumah and Terumah that is tamei on Erev Pesach must be burned separately, and they are not burned together.
So what is their dispute about? About Terumah teluyah, Terumah whose status is in doubt because it is uncertain whether it became tamei, which is to be burned together with Terumah that is tamei. In this case the burning stems from a matter of tumah - potential tumah only, to be sure, but still a matter of tumah:
Rabbi Eliezer: even in a case of doubtful tumah they must be burned separately, for one may not bring the doubtful Terumah to a state of certain and definite tumah.
Rabbi Yehoshua: they may be burned together, for even the Terumah whose status is in doubt is destined to be burned because of the potential tumah within it; and since that is so, it may be burned even though at that moment it will become definitely tamei.
But even Rabbi Yehoshua concedes, Rabbi Yosei stresses, that Terumah which is completely pure may not be burned on Erev Pesach together with Terumah that is tamei.
In summary: in this Mishnah Rabbi Meir derived from the words of Rabbi Chanina, the deputy Kohen Gadol, and Rabbi Akiva that pure Terumah is burned together with Terumah that is tamei on Pesach, since it is destined to be burned anyway on account of the chametz. Rabbi Yosei responded, "Einah hi hamidah," for there the item was tamei from the outset, whereas here the Terumah is entirely pure. He brought support for his position from the dispute between Rabbi Eliezer and Rabbi Yehoshua regarding Terumah teluyah, from which it emerges that with regard to fully pure Terumah all agree that it is not burned together with Terumah that is tamei.