Generally speaking, Pesach includes three positive commandments and four negative commandments; according to the Rambam there are five negative commandments. The positive commandments are:
- To eat Matzah on the first night of Pesach
- To retell the story of the exodus from Egypt on that night
- To eradicate [Heb. 'Tashbisu'] all Chametz (leavened food) or leavening agents [Lit. sour dough] on the 14th of the month
These are the four negative commandments:
- Not to eat Chametz (leavened food) all seven days
- Not to eat any mixture containing Chametz (leavened food or agents) all seven days
- Not to 'see' Chametz (leavened food or agents) all seven days.
- Not to be able to 'find' Chametz (leavened food or agents) all seven days
According to the Rambam there is also a prohibition against eating Chamtez on Pesach eve after [halachic] noon. Other authorities relegate this to the province of a positive commandment, that of 'Tashbisu' ('eradicate'). Now note that the prohibition of Chametz and Si'or (leavening agents or sour dough) are one and the same, as Si'or is that which makes foods leavened. One who eats Chametz or Si'or deliberately during the seven days of Pesach incurs the divine punishment of 'Kares' (spiritual excision), as the verse states: "...for any who eat Chamtez - will be cut off." If the act was done accidentally, one must bring a sin offering. It does not matter whether it is eaten or pureed and drunk. There is, however, no punishment of Kares for consuming a foods that have within them some mixture of Chamtez, it is only a Biblical prohibition.
(Rabbi Yehuda and Rabbi Shimon argue regarding Chametz 'before Pesach' [that is, after the sixth hour on Pesach eve, when it must have already been disposed of,] and Chemtz 'after Pesach' [that is, Chametz which was owned over Pesach]. Rabbi Yehuda posits that in both of these scenarios that Chametz is prohibited Biblically, and Rabbi Shimon disagrees [arguing that the prohibition is rabbinic]. In terms of the law, we have already found Rava to rule in Pesachim (30.1) as Rabbi Shimon regarding Chametz after Pesach. Most authorities also rule as Rabbi Shimon regarding the period of 'before Pesach'. The Rambam takes a different approach and rules as Rabbi Yehuda in the 'before Pesach' scenario.)
It is prohibited to derive benefit from Chametz on Pesach, as the verse states: "You will not eat Chametz" - that there will no permit to eat - meaning that there be no sequence of events that will permit Chametz to be eaten. In general, all benefits [that have some monetary value] lead to eating, as financial profits are often used to purchase one's daily meals (Rashi on Pesachim 21.2).
This line of reasoning is necessary for the opinion that understands the term "Lo Se-yachel" (do not eat) to refer actual eating. However, the Rambam has already ruled in the eighth chapter of forbidden foods, law 15, that the law is as the opinion that every usage of "Do not eat" in the Torah refers to any manner of usage, and prohibits eating as well as other uses. This definition would only be overruled were the Torah to specifically permits benefit, as is seen in the law of giving a non kosher animal carcass to a non Jew [who has no obligation to eat meat ritually slaughtered]. Here the verse states: "to the non Jew within your gates [community] you may give it and he will eat it." [and benefit will be accrued by this animal's sale] In regard to Chametz, the verse never permits benefit, so it remains prohibited.
(As to why the Rambam wrote in the beginning of the Laws of Chametz and Matzah the reason supplied by Chizkiah in the Talmud, that the prohibition to derive benefit arises from the construction of "Lo YeaOchel" ("לא יֵאָכל","Do not eat") which when modified by the vowelization of a "Tzereah" and a "Kohmatz" changes the meaning of the word so as to prohibits benefit (as is explained there in the Talmud), even though we actually rule in practice like Rabbi Avahu [and not Chizkiah] who does not require this exegesis - this is because the Rambam often quotes the simpler exegesis instead of delving into lengthy explanations, as is his custom in many places [as his work's primary goal is to lay out legal decisions, rather than record the analysis behind those decisions])
Here then is the language of the Rambam as found in chapter 1, law 8:
"It is prohibited to eat Chametz on the 14th from [halachic] noon and on, which is from the beginning of the seventh hour of the day. All who eat Chametz during this period receive lashes for the Biblical prohibition, as the verse states: 'Do no eat Chametz with it', meaning to say 'with the Pesach sacrifice'. Our received tradition informs us to interpret this as a prohibition not to eat Chametz from the time it is possible to offer the Pesach sacrifice, which is afternoon i.e., after midday."
Until here is the quote. Now even though the Pesach sacrifice was offered after the daily sacrifice was offered, which happened after midday, therby causing the Pesach sacrificed to be offered quite after midday, nevertheless this delay was only a practical consideration, but does not change the time when a Pesach can technically be offered. Certainly any Pesach offered immediately after the noon is valid, for the Torah also states of a Pesach that it is brought "after noon", only in practice the daily offering precedes it for the reason noted in Pesachim 59.1, that the verse also mentions the 'evening' in regard to the Pesach sacrifice, thus allowing the daily offering to rightly precede it. Furthermore, the daily offering has the advantage of the legal maxim that the more frequent occurrence always takes precedence ['Tadir v'Sheano Tadir, Tadir Kodem'] (Tosafos thereon).
If one had offered the Pesach before the daily sacrifice it is valid post facto, as the Rambam writes in chapter 1 of the Laws of the Pesach Offering. Since this is so, it is clear that when the Torah instructs one not to eat Chametz from the time of the Pesach offering it is referring to after midday, which is the primary time for the offering to commence. The Ra'avad and other authorities dispute this ruling, and their position is that there is no Biblical prohibition after midday [before the festival proper], and one is only afoul of the positive commandment of 'Tashbisu' ('eradicate'), as I have written.
The Rambam would quite naturally rule that Chametz after noon is likewise prohibited from benefit, for since the verse states "do not eat" the prohibition against benefit is part and parcel of that term, as he himself writes at the end of the eighth chapter of Forbidden Foods, which we mentioned in section 2 (see Kesef Mishna in the beginning of the first chapter).
To those that dispute his ruling there is no proibition against deriving benefit from Chametz until nightfall. This is the position of most of the authorities. However, note that there still is a rabbinic prohibition, as will be explained later in chapter 443. In terms of the legal nullification of Chametz, the Rambam treats the day before Pesach more leniently than Pesach proper in that the prohibition on Pesach concerns any amount of Chametz, no matter how minimal, whereas on the day before Pesach Chamtez can be considered nullified in 60 parts of non Chametz food ('Batel BiShishim') because at that time a Kares prohibition is not yet in effect, as will be explained in chapter 477 with the help of Heaven.
The Rambam writes the following in chapter 1, law 7:
"Eating any amount of actual Chametz on Pesach is forbidden Bibilically, as the verse states: 'Do not eat...'. Even so, one is only liable for 'Kares' or a sin offering if he were to eat a legally significant measure, which is a 'Kezayis' [olive size piece]. One who intentionally eats a less than a 'Kezayis' should receive lashes as a recalcitrant [this is rabbinic lashes]."
Until here is the quote. The Magid Mishna explains his reason to be that even a partial measure is forbidden Bibilically.
Now if this is so, a question arises: Why would the Rambam specify this rule by Chametz? Does it not apply as well by all forbidden foods? Furthermore, the verse he quotes implies quite the opposite, as the eating it refers to is that of a 'Kezayis' (Kesef Mishna). Some attempt to answer that since Chametz is only forbidden on Pesach one might assume that it does not fall under the rule of 'a half measure is Bibilically forbidden'. Therefore, they believe that the Rambam quotes the verse to stress that this prohibition on Chametz is Bibilical (Mishna L'Melech in the name of the RalNach).
However, this answer still appears insufficient, for in the beginning of the last chapter of Yoma (74.2) the Talmud mentions the law of half measures in regard to Yom Kippur, which also is just a one-day prohibition. Moreover, the verse still appears to imply quite the opposite.
There are others who state that the basic legal definition of eating is any amount, no matter how insignificant, whereas the olive-sized measurement [and the consequences dependent upon it] are exceptions to this rule received by Moshe at Mount Sinai (Mishna L'Melech in the name of the R'eim). This is even harder to understand, for all throughout the Talmud eating is defined as an olive-sized measure of foodstuffs. Now though the truth is that all measurements in Jewish law were received by Moshe at Mount Sinai [and are not recorded in the Torah per se], nevertheless it is clear that the law considers the term 'eating' in the Torah to be at least olive-sized, and therefore it is common for the Talmud to state 'eat' without having to specify 'an olive-sized piece'.
In my humble opinion, the explanation of the Rambam runs as follows: Chametz does have a Torah verse to prohibit partial measures. Afterall, Chizkiah's position in Pesachim (21.2) is that even the prohbition against benefit can be learned from "Do not eat", which when vocalized with a 'Tzirei' and 'Kohmetz' can be defined as 'benefit'. We also find Rabbi Avuhuh's position to be that all instances of the term 'eat' in the Torah denote benefit as well. The Rambam, for his part, rules as Rabbi Avuhuh, as I have previously written in section 2. According to Rabbi Avuhuh a question therefore arises: Why indeed does the Torah change the Hebrew vowelization of "Do not eat"? To this he would reply 'To include partial measures in the prohibition'. The intent of Rambam is then to point out that the change in the grammar are implying the amplification [including half measures].
If you should ask 'Why by Chametz, isn't the same true for all prohibitions?' Yes, but there is an important distinction. In Yoma there is a reason put forward for the prohibition of half measures: that these amounts may combine into a whole, legally significant measure. Therefore, were one to theoretically eat at the very end of the seventh day a half measure, and could no longer eat on Pesach any amount to reach a full measure, his act would end up being permitted. Now that the Chametz prohibition is sourced from an actual verse it would be prohibited in any event. Nevertheless, this act would not result in 'Kares' or a sin offering as it is only an amplification of a verse and not its intrinsic meaning. In this way it is like all other half measures found in other areas of the law.