פירוש על עבודה זרה 146:5

According to Hizkiyah, if forbidden wine falls in first to a vat, then the entire mixture is prohibited. The fact that water falls in after will not change the fact that the entire mixture is prohibited. But if the forbidden wine and water are first mixed together, then this mixture will be permitted, assuming that the water nullifies the forbidden wine. If this entire mixture now falls into a vat of permitted wine, the entire vat remains pure.
R. Yohanan says that even in the first case, the mixture is permitted.

R. Yirmiyah tries to pair Hizkiyah and R. Yohanan. According to R. Eliezer, the status of the dough depends on which dough falls in first. If prohibited dough falls in first, the whole mixture becomes prohibited. This is similar (to a certain extent) to Hizkiyah. But according to the rabbis, since there is not enough prohibited leaven to ferment the whole dough, the whole dough remains permitted. This is like R. Yohanan.

The problem with R. Yirmiyah’s statement is that Abaye said that R. Eliezer permits the mixture only if they first remove the prohibited leaven before the permitted leaven falls in together. But if they do not, then the prohibited leaven joins the permitted leaven to make the whole mixture prohibited. But this is not like Hizkiyah for Hizkiyah allows the mixture even if the prohibited wine is still there. Therefore, the equation of Hizkiyah and R. Eliezer falls apart.

The Talmud resolves that the argument is over a principle called “we see.” This refers to a case where a two of the same substance (wine), one permitted and one prohibited, fall into a permitted substance of a different type. Do we pretend (“do we see”) that the permitted substance of the same type does not exist such that the permitted substance of a different type can nullify the prohibited substance, or do we not “see” the permitted substance as if it does not exist. Hizkiyah holds by this principle and R. Yohanan does not.
Note that creating such an abstract principle facilitates comparing this case with other cases in the Talmud. It is one of the benchmark’s of the editors of the Talmud. They boil down one case to its essence, to allow it to be compared to other cases.

R. Asi asks R. Yohanan a question that seems to depend on whether we invoke the principle of “we see.” If we do, then the water would annul the terumah wine and anyone can drink this mixture? If we do not, then the mixture can only be drunk by kohanim.

Later, R. Yohanan answered R. Asi—indeed, we do invoke the principle of “we see” and the wine is permitted.

Here the question that R. Asi asked of R. Yohanan above is presented as a declarative statement.

Rav and Shmuel’s reading of the mishnah adheres closely to the simple meaning. Any time two of the same substance become mixed together, one permitted and one forbidden, for instance terumah wine with regular wine, the mixture will be prohibited no matter how small a quantity of prohibited substance. But if the substances are different, for instance lard in a pancake mixture, the mixture is prohibited only if the prohibited substance imparts a flavor. The last line of the mishnah indicates that this is true no matter what the prohibition.

R. Yohanan and R. Shimon b. Lakish are more lenient. In most cases, even with two of the same species, the mixture is prohibited only if the forbidden substance imparts taste [note that this is actually impossible to test. If non-kosher beef is mixed up with kosher beef, how could one tell whether the non-kosher beef imparts a taste]. The only exception is “tevel”—produce from which tithes have not been removed. And yayin nesekh.
The mishnah already referred to yayin nesekh. So the only extra thing it can add by saying “this is the general rule” is tevel. Again, this is a more difficult reading of the mishnah.

The baraitot in this section are word for word the same as the amoraic statements in yesterday’s section.

We can understand why R. Yohanan and Resh Lakish are stringent about yayin nesekh—the laws about idolatry are stringent. But why are they stringent about tevel—produce from which tithes and terumah have not been removed?
The answer is that the prohibition is the same as the permission. If a person separates one grain of wheat from a pile of wheat and declares it to be terumah, the entire pile permitted. So too one grain of wheat can make the entire pile prohibited. In other words, one grain has significance, both to make a pile of grain (or any type of terumah, such as a vat of wine) permitted, so too it has the power to make an entire pile prohibited, if one prohibited grain falls in.
Finally, the Talmud cites another baraita that supports the idea that tevel prohibits a mixture in the smallest amount only when it is of the same species. This baraita accords with R. Yohanan and Resh Lakish.