Rambam - 1 Chapter a Day

With this specification, the Rambam excludes food which cannot be purified through immersion.

See Chapter 4, Halachah 1.

I.e., immersion in a container is not acceptable even if the container is large enough to hold the required amount of water.
Sefer HaMitzvot (positive commandment 109) and Sefer HaChinuch (mitzvah 175) include immersion as one of 613 mitzvot of the Torah. Sefer HaMitzvot, op. cit., clarifies that the intent is not that an impure person has an obligation to purify himself. {The commentaries explain that the Rambam agrees with the opinion that there is no mitzvah for a woman to purify herself after her nidah impurity). Instead, the mitzvah is that a man or woman who is impure can regain purity only through immersion in a mikveh.
See also Hilchot Metamei Mishkav UMoshav which states that after a zav, a zavah, a nidah, and a woman after childbirth contract impurity, they remain impure even if they remained several years without an impure emission, if they did not immerse in a mikveh. And Hilchot Issurei Bi’ah 11:16 states:
A woman does not ascend from her state of ritual impurity... until she immerses herself in a mikveh that is halachically acceptable while there are no substances intervening between her flesh and the water.... If, by contrast, she washes in a bath — even if all the water in the world passes over her — her state is the same after washing as before washing.... For there is no way of ascending from a state of ritual impurity to one of purity except through immersing in the waters of a mikveh, a spring, or a sea which is like a spring, as will be explained in Hilchot Mikveot.

Leviticus 14:9, 15:13; Numbers 19:7-8, et al.

Leviticus, op. cit., Numbers 19:8, 10, et al.

As the Rambam proceeds to explain, the intent of the verse is that the person must immerse his entire body, including his hands. As stated in Halachah 10, the prooftext also implies an exclusion: Just as the hands are visible, so too, it is only the visible parts of the body that must come in contact with water. Parts of the body that are not openly visible need not come in contact with the water.

I.e., that is where these principles are elaborated upon at length.

I.e., immerse in a mikveh.

I.e., even after immersion, the person’s impurity does not depart until nightfall.

The Rambam uses the expression binyan av, which is one of the Thirteen Principles of Biblical Exegesis. Literally, the term means “a building of a father.” The intent is that the Torah states a law regarding a particular subject. That subject then serves as a prototype and the same law is applied to all similar subjects. Similarly, in the case at hand, the prooftext states that a person or a k’li that contracted impurity through contact with the carcass of a crawling animal can regain purity through immersion. From that one instance, we learn that all impure people and objects can regain impurity through immersion unless the Torah explicitly states that other means of purification are necessary.
(The commentaries note that in Hilchot Issurei Bi’ah 4:3 the Rambam mentions a different verse to serve as a binyan av for this concept. It is possible to explain that there he is speaking about the purification of people, while here, the intent is both people and keilim.)

See Hilchot Keilim, chs. 18-19, with regard to the purification of keilim through breaking them.

The earth and anything attached to it are not susceptible to ritual impurity. Nevertheless, since the k’li was impure before it was attached to the earth, it remains impure after it was attached.

This refers to an earthenware container that has already contracted impurity. If it has not yet contracted impurity, there is room for leniency (see Merkevet HaMishneh).

Since the raw material from which glass keilim are made is sand (a type of earth), our Sages considered glass keilim like earthenware (Shahbat 15b).

As stated in Hilchot Keilim 23:1:
A mafatz is a mat made by interweaving cords, reeds, grasses, or the like. A mafatz is not one of the keilim mentioned by the Torah. Nevertheless, it is susceptible to the impurity associated with the support of a zav (midras impurity) according to Scriptural Law. [The rationale is that Leviticus 15:4] states “All surfaces on which one lies,” expanding the category [of articles susceptible to impurity. A mat is thus included, because] it is fit to lie on and is indeed made for that [purpose]. Similarly, it contracts impurity from contact with a human corpse and other sources of impurity according to Rabbinic Law, like all other flat wooden keilim, as we explained. This is a great general principle: Any entity that is susceptible to the impurity associated with the support of a zav is susceptible to other types of impurity.

The Ra’avad elaborates, explaining a slightly different perspective than the Rambam.
According to the Ra’avad, simple wooden keilim can be purified through immersion. It is only mats that cannot be purified, because they are not made of wood and are not considered as keilim at all. [See the gloss of the Merkevet HaMishneh who explains that the difference of opinion between the Ra’avad and the Rambam corresponds to that between Rashi and Tosafot (Shabbat 84a).]

By being tom, it is considered comparable to a broken k’li which is no longer susceptible to impurity.

This is the minimum measure for such an article to contract ritual impurity (Hilchot Keilim 23:3).

In Hilchot Mechusrei Kapparah 2:1, the Rambam states that this refers to a man who experiences "[a discharge of] sperm that is released because of an internal sickness that affects the organs [of the body] in which it collects,” a condition somewhat similar to, but by no means identical with, gonorrhea. See Leviticus 15:2.

I.e., for a zav, immersion in a mikveh is not sufficient. See Chapter 9, Halachah 5.

As stated in Hilchot Parah Adumah 6:1, 9-13, this term refers to water that flows from a stream or river that originates in the earth. A stream of rainwater is not acceptable.

“[A woman] who experienced [uterine] bleeding for three consecutive days at a time other than the days when she usually menstruates” (Hilchot Mechusrei Kapparah 1:6; Leviticus 15:25).

Even garments that contracted impurity from a zav.

They may, however, immerse or be immersed in a stream as well. See Hilchot Issurei Bi’ah 11:16.

See examples of this concept in Hilchot Issurei Bi’ah, ch. 6, Hilchot Metamei Mishkav UMoshav, ch. 5, and Hilchot Parah Adumah 1:14.

I.e., a woman who menstruates is impure for seven days.

As stated in Leviticus 12:2, 5, a woman who gives birth to a male is impure for seven days and one who gives birth to a female is impure for fourteen days. Included in this category is also a woman who miscarries. Afterwards, the woman immerses to regain purity.

After the entire span of days when they are impure has passed.

Hilchot Issurei Bi’ah 4:6. At present, unless there are extenuating circumstances involved, all women immerse at night [see Hilchot Issurei Bi’ah 6:7; Shulchan Aruch (Yoreh De’ah) 197:3-4].

The Ra’avad objects to the Rambam’s ruling and maintains that any woman who need not count seven “spotless” days may immerse from nightfall onward. The Kessef Mishneh maintains that the Rambam would accept the Ra’avad’s statements and mentions only a person with a seminal emission, because such a person is explicitly mentioned in the Torah.

I.e., the Jewish day begins at nightfall. If a man had a seminal emission shortly after nightfall, he may immerse immediately. His status is then that of a t’vul yom (see note 33) until nightfall the following day.

“Toward nightfall” implies before nightfall. The verse is thus indicating that if the person had a seminal emission during the day, he may immerse during the day. Extending that logic, if he has a seminal emission at night, he may immerse immediately afterwards.

When he becomes pure. Between immersion and nightfall the following day, his status is that of a t’vul yom, i.e., he is still considered ritually impure and is forbidden to partake of terumah or sacrificial foods or enter the Temple. The laws regarding the impurity he imparts are, however, somewhat more lenient.

If even the smallest part of a person’s body is not immersed, the immersion is not acceptable.

So there are no intervening substances between their bodies and the water. See Halachah 12.

I.e., even if they immerse their entire bodies, but do so limb after limb, the immersion is invalid.

Thus even if one hair is not submerged in the water, the immersion is invalid. See also Chapter 2, Halachah 15.

This applies to loose-fitting garments worn in the Talmudic era and in the Rambam’s time. There are questions whether this law applies with regard to garments that cling to the body tightly. See Siftei Cohen, Yoreh De’ah 198:56.

I.e., her immersion is accepted after the fact. Moreover, in certain extreme situations, e.g., when there are concerns of modesty, she may immerse in her clothes as an initial preference.

As the Rambam states at the conclusion of ch. 11, this is one of the factors that distinguishes the laws of purity as a chok, a decree above rational comprehension.

There were individuals who would frequently partake of terumah (or consecrated foods). Hence, they were stringent and even when preparing and partaking of ordinary foods would follow the stringencies that apply to terumah (or consecrated foods; see Rashi, Chulin 34a). More discussion concerning this matter can be found in Hilchot She’ar Avot HaTum’ah 9:11, 13:2, et al.

See also Chapter 9, Halachah 17.

The Shulchan Aruch (Yoreh De’ah 198:48) quotes the Rambam’s ruling as halachah. The Rama notes that there are more stringent views and states that, as an initial preference, it is proper that a woman immerse again with the proper intent in such circumstances.

In which water flows from a natural stream.

The Kessef Mishneh interprets the wording used by the Rambam (taken from the Tosefta, Mikveot 8:1) as implying that the person must be checked to see that in fact the water covered his entire body.

Initially, in his Kessel Mishneh, Rav Yosef Caro quotes a responsum of Rivosh (Responsum 293) who explains that such conduct is undesirable, because it will create an impression that one is jumping into the water to cool off and not with the intent of immersing himself for the sake of purification. Afterwards, he offers a second explanation, based on the tractate Derech Eretz Zuta, ch. 10: this is speaking about a mikveh with exactly 40 se’ah and if the person jumps in, it is likely that some of the water will splash out and the mikveh will lack its minimum measure. This is the rationale Rav Yosef Caro quotes in his Shulchan Aruch (Yoreh De’ah 201:62). See also Kallah Rabati, ch. 10, which offers other rationales for this and the following laws.

The Rivosh (Responsum 293) states that the rationale is that since he is not immersing in the desired fashion, it appears that he is not immersing with the intent of purifying himself. In his Kessef Mishneh and his Shulchan Aruch (Yoreh De’ah 201:62), Rav Yosef Caro explains that this also is speaking about an instance where the mikveh has only 40 se’ah. It is possible that the first immersion was not performed properly, because he was relying on the second one and it is possible that the second immersion was not acceptable because some water flowed out of the mikveh during the first immersion (Turei Zahav 201:76).
It must be noted that it is customary in many communities for women and men to immerse more than once for a multitude of halachic and mystic reasons. See Ba’er Heiteiv (Yoreh De’ah 200) who states that a woman should immerse once before reciting the blessing and once, afterwards.

Here also the same two approaches are taken. The Rivosh explains that this creates an impression that one is not immersing for the sake of purification. Rav Yosef Caro [in his Kessef Mishneh and Shulchan Aruch (Yoreh De’ah 198:29)] states that this is speaking about a mikveh with just 40 se’ah and we are afraid that the insertion of the other person’s hand will cause some water to flow out of the mikveh. See also Chapter 2, Halachah 11, which states that a person should not hold onto a person who is immersing unless he dips his hand in the mikveh water first.

E. g., his mouth, the inside of his ears, nose, or the like [the Rambam’s Commentary to the Mishnah (Mikveot 8:5)].

For this reason, a woman must clean her teeth before immersion [Shulchan Aruch (Yoreh De’ah 198:24)]. Similarly, a bandage placed in these parts of the body disqualifies an immersion (ibid.:43).

Nidah 66b.

If a woman did not rinse these portions of her body before immersion, but no intervening substance was discovered afterwards, the immersion is acceptable [Shulchan Aruch (op. cit.:25)].

When performing these activities, her limbs are spread slightly, enabling water to reach under her arms and between her legs. See Hilchot Tum’at Tzara’at 9:12; Shulchan Aruch (op. cit.:35).

Or at a beach or any other public place.

Nidah 67a relates that the father of the Sage Shmuel would erect mats to enable his daughters to immerse in the harbor. Other commentaries explain that the mats served different purposes.

It is suspected that out of fear, she will not immerse properly. This is one of the interpretations given in Nidah 66b. Another interpretation is that our Sages forbade such an immersion lest a woman sit on a wooden bench in a bath and immerse herself. The difference between these interpretations is the ruling after the fact, i.e., if a woman immersed herself while standing on such an article. According to the first interpretation, if the immersion was carried out properly, it is acceptable. According to the second interpretation, if she immersed when standing on wooden keilim whose outer surfaces are susceptible to impurity, there is reason to say that the immersion is not acceptable. (Since the outer surface of an earthenware container is not susceptible to impurity, if she immersed while standing on such a surface, the immersion is certainly acceptable after the fact.) Rav Yosef Caro discusses this matter at length in his Kessef Mishneh. In his Shulchan Aruch (Yoreh De’ah 198:31), he rules that, after the fact, even if a woman stood on a wooden k’li, the immersion is acceptable. The Turei Zahav 198:31 and the Siftei Cohen 198:43 discuss this ruling and mention more stringent views.

This halachah serves as an introduction to the following two chapters. In Chapter 2, the Rambam discusses substances that intervene between a person’s flesh and the waters of a mikveh. And in Chapter 3, he discusses substances that intervene between a k’li and the water.

When stating these laws, the Shulchan Aruch (Yoreh De’ah 198:1) uses slightly different wording, stating: “If the way of people is that sometimes they object, it is considered as an intervening substance.” The implication is that the ruling is not dependent on the feelings of the person alone, but it is dependent also on the norm accepted in the society. Thus if there are people who object to such a substance and the person objects, it is considered as an intervening substance. Similarly, if most people object, but the person does not object, it is considered as an intervening substance. It is only when only a few people object and this person does not object that it is not considered as intervening (Turei Zahav 198:2).

Nevertheless, as an initial preference, one should not immerse even when the substance is of the type to which no one would object.