Mishnayos Taharos Perek 2 Mishnah 2

האוכל אוכל ראשון – his body is made first degree [of ritual impurity] so that his contact is second-degree [of ritual impurity]. And the person who eats food that is second-degree [of ritual impurity], his body is made second-degree [of ritual impurity], but if he came in contact with heave-offering/priest-due, he invalidated it, these are from the eighteen matters that were decreed on that self-same day (i.e., when Rabban Gamaliel was deposed as President of the Sanhedrin described in Talmud Berakhot 28a – see the Bartenura commentary to Tractate Shabbat, Chapter 1, Mishnah 4 for the full list of the eighteen decrees). And specifically, that he consumed half of a loaf, which is an egg and a half worth according to Maimonides and two eggs’ worth according to Rashi. And this is the measurement of food to make the body unfit.

רבי יהושע אומר האוכל אוכל ראשון ואוכל שני נעשה גופו שני – for he holds that we don’t compare for him the one who eats to be like the food itself. For that which compares the person who consumes second-degree [of ritual impurity] food to be second degree [of ritual impurity] and not third-degree [of ritual impurity, because since we found that second degree [of ritual impurity] makes someone second-degree [of ritual impurity] through liquids, for the liquids which were defiled by someone with second-degree [of ritual impurity] are made first degree [of ritual impurity] and a person who comes in contact with them is second-degree [of ritual impurity], but if we would compare to him who consumes food that is second-degree [of ritual impurity] becomes third-degree [of ritual impurity], it would not invalidate the heave-offering/priest’s due, for something that is third-degree [of ritual impurity] does not make something fourth-degree [of ritual impurity] regarding heave-offering/priest’s due, but the Sages decreed on someone who consumes food that is second degree [of ritual impurity] that he would invalidate the priest’s due/heave offering, because sometimes he consumes ritually impure foods and casts liquids of heave-offering into his mouth while the ritually impure foods are in his mouth and invalidates them, therefore, even though that when eating foods that are first-degree [of ritual impurity], we don’t compare him to one who eats like the food itself, that by consuming food that is second-degree [of ritual impurity] we compare him to be second-degree [of ritual impurity] like the food, in order to invalidate the heave-offering/priest’s due.

שלישי שני לקודש – a person who eats food hat is third-degree [of ritual impurity], his body becomes second-degree [of ritual impurity] for Holy Things, and he makes third-degree [of ritual impurity] and fourth degree [of ritual impurity] with Holy Things.

ולא שני לתרומה – for if he came in contact with heave-offering, he did not invalidate it, but however, he is prohibited to consume heave-offering, and this is proven in the Gemara in Chapter Two of [Tractate] Hullin [folio 37a].

בחולין שנעשו על הרת תרומה – meaning to say this food is food of first-degree [of ritual impurity] and one who consumes food of second-degree [of ritual impurity] and third-degree [of ritual impurity] that we taught in our Mishnah, we are speaking about unconsecrated produce that was made on the purity of priest-due/heave offering, for mere unconsecrated produce does not have a third-degree [of ritual impurity].