Rambam - 1 Chapter a Day

If, however, a person touches a secondary derivative of impurity, his hands do not contract impurity.

There is a difference of opinion among the Rabbis regarding which joint is intended. Some maintain the intent is the place where the fingers are joined to the hand. Others maintain that the intent is the wrist, the place where the hand is joined to the arm. See the notes to Hilchot Mikveot 11:4.

Generally, a primary derivative of impurity does ‘not impart impurity to humans.

This stringency is one of the eighteen decrees instituted on the renowned day when the students of the School of Shammai outnumbered the students of the School of Hillel (Shabbat 13b).

I.e., any primary source of impurity—whether of Scriptural or Rabbinic origin—that imparts impurity through touch, as stated in Chapter 6, Halachah 12.

In both of these instances, one might think that the hands would be considered as primary derivatives of impurity.

A sacrifice deemed unacceptable because of a disqualifying intent regarding the time when it was to be offered or its meat eaten. Pesachim 85a explains that the Sages suspected that there might be priests who would desire to disqualify the sacrifices they were offering by having such an intent in mind, because of a personal feud with the person bringing the sacrifice. Hence, the Sages decreed that such an offering would make the priest's hands impure. Because of the extra difficulty this impurity would cause the priests, they would refrain from offering the sacrifices with such an intent.

Sacrificial meat left over beyond the time when it was supposed to be eaten. The Sages suspected that the priests might be lazy and not partake of all the sacrificial meat. Hence they decreed that any leftover meat would be considered impure so that, rather than be inconvenienced by the impurity, the priests would not leave over any meat (ibid.).

Usually, the flour of the meal offering becomes fit to contract impurity, because it is touched by the oil that is mixed with it. Nevertheless, here, the Rambam is speaking about grains of flour that become stuck together and hence were not touched by the oil. Even so, since they were part of a sacrifice, the cherished nature of the sacrificial offerings makes them fit to contract impurity. See Chapter 12, Halachah 13. See also Halachah 14 of that chapter which appears to contradict this ruling.
As the Ra’avad and Rav Yosef Corcus note, there is a difference between these portions of flour and piggul and no tar. Piggul and no tar are impure even when they do not come in contact with a source of impurity. These portions of flour, by contrast, must contract impurity through contact with a source of impurity.

Pesachim 120b discusses this issue, mentioning that there were Sages who maintained that since partaking of an olive-sized portion of these foods makes one liable for lashes, that amount of these sacrifices would impart impurity. Since this impurity is a Rabbinic issue, the Rambam, however, follows the opinion of the Sages who require the larger, egg-sized measure (Kessef Mishneh).

I.e., if there is half an egg-sized measure of piggul and half of notar, we do not say that there is an egg-sized measure of impure food.

I.e., if an egg-sized measure comprised of two types of food contracts impurity, it can impart that impurity to other entities.

See Hilchot Tum’at Ochalin 4:1.

I.e., the bare bone, even if there is no meat on it.

Note the contrast with the law that applies to the thigh bone of an animal carcass or the thigh bone of the carcass of a crawling animal which does not impart impurity unless the bone has been opened (see Chapter 2, Halachah 11; Chapter 4, Halachah 9).

I.e., the marrow of the bone which is fit to contract impurity. See also Hilchot Tum’at Meit 2:5 which states that similar laws apply to the thigh bone of a corpse. There, however, the impurity stems from being under the same structure as the corpse. Here, there is a further stringency, because the impurity stems from touch.

I.e., sacrifices of the most sacred order may be eaten only inside the Temple Courtyard and sacrifices of lesser sanctity may be eaten only within the city of Jerusalem. Whenever sacrificial meat is taken beyond these boundaries, it is disqualified. See Hilchot Ma'aseh HaKorbanot 11:6.

The Sages (Pesachim 85a) debated whether it was desirable to impose this stringency on the priests or not and left the matter unresolved.

See Chapter 14, Halachah 1.

The meat of a given Paschal sacrifice may only be eaten in one house. It may not be taken from house to house. See Hilchot Korban Pesach 9:1-2.

As stated in Hilchot Korban Pesach 2:1, the Paschal sacrifice is not eaten alone, but together with a group of family and friends. As such, we assume that the group will be vigilant and prevent the meat from being taken outside the home in which it was being eaten. Accordingly, the Sages saw no reason to decree that such meat should be considered as impure.
In his gloss to the Mishneh Torah, Rabbi Akiva Eiger emphasizes that these laws apply with regard to a Paschal sacrifice that is taken outside the house where it is being eaten. If, however, the Paschal sacrifice is taken out of Jerusalem, like other sacrifices, there is reason to say that it does impart impurity to the hands of one who touches it.

Many Rabbinic safeguards were not applied in the Temple, as stated in Hilchot Shabbat 21:27, Hilchot Korban Pesach 1:16, et al.

And later entered the Temple.

Causing it to be considered impure.

Chapter 11, Halchot 3-4, 7.

By pouring a revi’it of water over it from a utensil; see Hilchot Mikveot, ch. 11.

If he does not touch the other hand, it is sufficient to immerse the one that contracted impurity (Kessef Mishneh).

The phrase is quoted from Shabbat 14a-b. The Rambam’s Commentary to the Mishnah (Taharot 7:8) interprets that phrase as meaning that a person is careless with his hands and touches many entities with them. The Sages were concerned that perhaps one would touch an impure entity.

Hillel and Shammai (Shabbat, loc. cit.).

As the Ra’avad mentions, this is the source of our practice of washing before partaking of bread. As related in Chulin 106a, Rabbi Elazar ben Azariah extended the decree even to unconsecrated foods. His decree was also intended to remind the priests to keep their hands ritually pure while partaking of terumah. It was, however, instituted not only for priests, but for the nation as a whole (even though non-priests may not partake of terumah), so that it would be a universally accepted practice.
Even after the destruction of the Temple, when it is no longer possible to practice ritual purity, this mitzvah was continued in the hope that the Temple will soon be rebuilt, and the priests will resume their previous obligation (Shulchan Aruch HaRav 158:1; Mishnah Berurah 158:1).

It is forbidden to burn terumah and consecrated foods that are pure according to Scriptural Law. For that reason, generally, terumah is not burnt after it comes in contact with impurity of Rabbinic origin. Nevertheless, in this instance, our Sages ordained that the terumah should be burnt (Shabbat 17b).

Because of the severity involved in doing so.

As explained in the notes to Chapter 11, Halachah 9, there were certain pious individuals who maintained ritual purity in all situations and would partake of all foods, even ordinary foods, only when they conformed to the stringencies of ritual purity associated with sacrificial foods. Others were slightly less stringent and relaxed the standards somewhat, keeping only the stringencies associated with terumah which are slightly more lenient than those associated with sacrificial foods.
Since these observances represented merely a personal stringency and were not required by law, our Sages were concerned that a person might be somewhat lax and loosen his vigilance to a certain degree.
It must be noted that the leniency mentioned here is accepted by the Rambam with regard to partaking of bread in the present age (Hilchot Berachot 6:18). There are other authorities who do not accept this leniency. In his Kessef Mishneh to those halachot, R. Yosef Caro offers a rationalization for the Rambam’s ruling, explaining that our food is impure regardless. Hence there is no rationale for extra stringency. Nevertheless, in his Shulchan Aruch (Orach Chayim 163:1), he accepts the Rambam’s leniency only when water is not easily available.

Chapter 7, Halachah 5.

From the Rambam’s statements in his Commentary to the Mishnah (Introduction to Seder Taharot and Zavim 5:12), it appears that this applies only to foods that are terumah.

I.e., although our Sages’ decree was instituted lest a person may have touched a source of impurity, it is ultimately no more than a Rabbinic safeguard.
The Ra’avad differs with the Rambam (see also Chapter 7, Halachah 2, and notes) and maintains that such liquids do impart impurity to keilim. The Kessel Mishneh explains the basis for the Rambam’s ruling.

Since the impurity stems from a Rabbinic decree, after immersion, the person regains purity immediately. There is no necessity to wait until nightfall (Chapter 9, Halachah 1).

This is speaking about terumah, for there is no concept of a tertiary derivative of impurity with regard to ordinary foods.

Even though the impurity of foods is of Scriptural origin, they do not impart impurity to the person according to Scriptural Law.

Hilchot Terumah 7:3.

The Rambam uses the expression k’chatzi p’ras, “half of a p’ras.” A p’ras is half a loaf of bread, so that half a p’ras is a quarter of a loaf of bread. A loaf of bread is equivalent to the volume of six eggs so this measure is equivalent to the volume of an egg and a half.

Approximately 57 grams according to Shiurei Torah, 100 grams according to Chazon Ish.

86 cc according to Shiurei Torah, 150 cc according to Chazon Ish.

Even if they are of different types entirely, as stated in Hilchot Tum’at Ochalin 4:3.

Causing that person to be considered as a secondary derivative of impurity.

This measure, k’dei achilat p’ras, is the measure of time associated with mitzvot and prohibitions associated with eating, as stated in Hilchot Ma’achalot Assurot 14:8, et al. Shiurei Torah mentions several different opinions from between four minutes until nine minutes for this measure.

See parallels in Hilchot Sh’vitot Asor 2:4; Hilchot Ma’achalot Assurot, op. cit.; Hilchot Terumot 10:3.

The Radbaz (Responsum 1554) notes that in Hilchot Sh’vitat Asor, op. cit., the Rambam mentions the measure of “the time to drink a revi’it,” a much smaller amount of time. He explains that since only a Rabbinic decree is involved here, our Sages ruled stringently, so that the matter would not be treated lightly.

The immersion is not significant. An immersion cleanses a person of impurity when he was deemed impure. In this instance, however, the immersion has no effect, because the person was never deemed impure.

A person who is a primary derivative of impurity does not impart impurity to other people through touch. Nor does the woman impart impurity to her son because she nursed him, due to the reasons the Rambam proceeds to explain.

A pregnant woman who desires foods may endanger her fetus if she does not satisfy her hunger. Perhaps even her own life would be endangered. Therefore certain leniencies are granted in such situations (see Hilchot Ma'achalot Assurot 14:14; Hilchot Sh'vitat Asor 2:9). Similarly, in this instance, they were concerned that if the woman knew that she would be deemed impure, she would refrain from partaking of the food and endanger herself.

Apparently, the intent is that she is permitted to partake of many portions, each one being less than the minimum measure. Although ultimately she will have eaten more than the minimum measure, our Sages relaxed their decree because of the danger involved.