1. Explanation:

There are a number of exceptions to the general rule that an issur may be nullified by a heter. For instance:

• Something of importance (dover chashuv),
• Something that is generally sold by number rather than weight (dover s'b'minion)
• A portion fit to be served to a guest (chaticha r'oui lhiscoved)
• Chometz on Pesach

All these cases follow their own specific rules. They are exceptions to the general rule and are not nullified in sixty or in any other rate. Rather, in general, they are never nullified, not even in a thousand.

One of these exceptions is "something which will become permitted" (dover sh'yash lo materin). That is to say, that although there is an issur now, that issur will become mutar at a later time, or in other circumstances. The simple reason (svora) why Rabbis do not permit the mixture to be eaten now, under the rule of bitul (nullification) is: It is better to wait and eat something that is completely heter, when the issur becomes mutar, rather than to rely on the factor of bitul and eat what does, in fact, contain an issur, albeit an issur that is mvutal (nullified).

2. Source:

The source of this rule is found in the Gemora, Betzeh 3:

• An egg which is laid on Shabbat or Yom Tov may not be carried, nor used to cover a vessel, nor may the legs of a bed be supported upon it. A vessel, however, may be placed over it to keep it from breaking. In a doubtful case (sofek) the egg is prohibited. If it is mixed, even with a thousand, they are all prohibited.
• (The Gemora continues.) According to the views which hold that the issur of an egg laid on Shabbas or Yomtov is Rabbinical (issur d'rabbonon), why is the sofek egg forbidden? We should apply the rule: "a doubtful case dealing with a Rabbinical prohibition is permitted (sfaka d'rabbonon l‘kulah).
• On the other hand, if we maintain that the sofek is not whether the egg was laid on YomTov or not (which according to the Gemora at this point would be an issur d'rabbonon), but rather a sofek whether the egg is treife or not (whether it came from a chicken that was treife), then the issur is from the Torah (d'oraissah). Under these circumstances, the normal rules of bitul apply to such issur and therefore, we do understand why the issur is not botel in a thousand.
• Rav Ashi explains that we are dealing with a sofek of an egg which may have been laid on YomTov. He holds that even a sofek issur d’rabbonon,is not botel in a dover sh’yash lo materin (abbreviated "dsyl"m").

Certain rules can be clarified from this Gemora:

The rule dsyl"m applies even where the issur is d'rabbonon and even where there is a sofek issur.

The Rashba (brought by the Tor, 102) explains that this rule only applies if it is certain that the issur will become mutar, or that it is in the hands of the person to make it mutar. The egg, for example, will certainly become mutar after Shabbas or Yomtov. The treife, on the other hand, may possibly be mutar - for instance, if the chicken lays another egg or lives for twelve months (which proves that it was not treife) and therefore, the egg was also not treife. However, this eventuality is not certain, nor is it in the hands of the owner to determine. We therefore do not apply the stringency of dsyl"m to a treife and the heter of bitul will apply to it.

3. Clarifications:

We must make four clarifications to this rule:

I. Do we only apply the rule where the heter will come about inevitably or without the expenditure of money? If, however, is necessary to expend money to produce the heter, do we say the rule does not apply?

For example, in the case of a cooking vessel which has absorbed treife (blius) and is then used with other heter foods; do we allow the treife to be botel in the majority (rov) of heter or do we say, on the contrary, that the issur is a dsyl"m? Do we rule that it may not be used because it is a dsyl"m since the vessel can be made mutar by way of koshering by immersion in boiling water (hagala)?

The Rashba and Rabbeinu Tam hold that since the heter only comes  by the expenditure of money (spent in making a fire to kosher the vessel), we do not apply the rule dsyl"m and therefore we may rely on the bitul of the vessel in a rov.

The Maharil (brought by the Shach) states that in such a case since the expenditure required for hagala is slight and therefore dsyl"m applies and the vessel may not be used.

II. Do we apply the rule in a case where the issur will become heter only when it will be spoiled.

The Mordechai holds that in such a case dsyl"m does not apply and we may rely on bitul. The Shulchan Oruch brings down this view as "there is one that says" which implies that there is another view.

One explanation of "the other view" is that of the Or Zerua. The Or Zerua asks: Do we apply dsyl"m in a case of a sofek in din? For example, where the question of whether there is an issur is left open by the Gemora when it states Taku (the case is to be decided by Eliyahu HaNave). Do we say if such a sofek issur is mixed with heter, since Eliahu may rule this sofek to be muter, we say this is case is dsyl"m, or are we allowed to m'vatel the issur, because it is not certain that Eliahu will do so.

The Or Zerua answers that we may mvutel in either case. If Eliahu will say that the sofek is muter, then his decision will reveal that it always was mutar (a gilui milta). Therefore, we may eat the mixture freely for there is no issur there. If on the other hand, he will say that the sofek is osser, then it equally follows that it was always osser. Therefore, it may be eaten now under the general law of bitul.

The later authorities (Achronim) point out that since the Or Zerua did not explain that in such a case we do not apply the rule of dsyl"m because were we to wait it is possible that the mixture would spoil in the meantime, implies that he does not hold such a view.

This, therefore, is the opposing view referred to by the Shulchan Oruch. It is also possible that the opposing view is the Rambam as explained in the next sub-paragraph.

III. The next clarification is whether the rule applies when the issur, after it has become heter, will once again return to be issur.

For example, mukzeh is osser on Shabbas or YomTov, but will become muter on Yom Chol. It again become issur on the next Shabbas or YomTov. Similarly, Chometz is osser on Pesach. It will be muter after Pesach and will return to it issur state on the next Pesach. Do we say that Chometz on Pesach is never botel because of dsyl"m? Do we only consider the fact that it will become muttar after Pesach or does the rule only apply if the issur will become permanently muter, which is not the case of Chometz on 'Pesach.

The difference of opinion (machlocus) in the Rishonim can be explained by understanding the different reasons of why we hold that Chometz on Pesach is osser in any quantity (a mashahu) and is never botel in a rov.

1. The Rambam holds the reason is that because of dsyl"m the chometz is always osser and may never be botel. From here we see that the Rambam holds that the rule applies even though the issur will return to issur after a period of heter. It may be inferred that he also applies the rule even though the issur will be spoiled in the meantime (see above)
2. The Rosh, on the other hand, gives two other reasons why the Rabbis made a decree that Chometz on Pesach is osser in a mashahu and not botel: 1] First, in the Gemora Pesachim, because of the strictness of the issur which is punishable by death from the Heavens (Koras). 2]   Secondly, in the Gemora Avodah Zorah, because people do not naturally remove themselves from Chometz which is something which is muter during the year and osser only on Pesach. The Rabbonon are therefore strict because it is necessary to create a greater separation.

The Orech HaShulchan explains that these two reasons are not self-contradictory. The principal reason for the gezareh is the strictness of the issur. However, the fact that the Rabbonon gozer on a mashahu in a mixture of Chometz, and not on other issurs which have koras, for example blood, is because of the second reason. That is, in the other issurim people already separate themselves from the issur, which is not the case in Chometz.

In any event, it is clear that the Rosh does not hold that the reason a mixture of Chometz on Pesach is not mvutel is because of dsyl"m. Presumably because he holds the rule does not apply when the issur will return.

The following halachic differences (nufka minos) can be found between the Rosh and the Rambam:

1. Chometz on erev Pesach is osser but does not have a din of koras. According to the Rambam the mixture of Chometz on Erev Pesach is osser in a mashahu - dsyl"m. According to the Rosh, not so, there is no gezera because there is no issur of koras and dsyl"m does not apply because the issur will not become muter permanently.

2. In the case of Mukzeh, the Rambam will hold dsyl"m applies and the Rosh will say it does not.

The Rambam holds dsyl"m in the laws of Pesach, even though it may spoil. It follows that he does not hold this is factor removes the din.

IV. Does the din of dsyl"m apply only where the substances of heter and issur are the same kind (min b’mino), or even where they are different kinds (min b’aino mino)? Does it only apply when the substance of the issur is in the mixture (b’eyn), or even when there is just a taste (ta'am) of the issur? Since the source of the rule in the Gemora Betzei is a case of wood that fell into other wood, where the issur is both min b'mino and b'eyn, do we limit the rule to such cases and any other case we allow the rule of bitul to apply, or do we apply the rule of dsyl"m stringently even in cases of min b’aino mino and where there is a ta'am of the issur?

The Shulchan Oruch holds that the rule only applies to min b'mino. The Rama further adds that the rule only applies to where the actual substance of the issur is present, but if there is only ta'am, the rule does not apply and the bitel takes affect. On this last din, the Rashal disagrees and applies dsyl"m even to taste.

4. Explanation:

The rule of dsyl"m can be explained in one of two ways :

1) According to Rashi, as we have said, the reason is that the Rabbis do not wish to rely on the heter of bitul in a case where the issur will pass and therefore the need for the bitel will also pass and the mixture will be completely muter.

2) According to the Ran (Nedarim 52) the reason is different:

There is a machlocus between Rabbi Yehudah and the Rabbis as to whether min b'mino is botel or not. According to Rabbi Yehudah it is never botel. According to the Rabbis it is botel in a rov. Rabbi Yehudah's source is from a posuk in Parsha Achrai Mos. In the Yom Kippur service, the Cohen Gadol is commanded to bring the blood of the ox and of the goat (Rabbi Yehudah holds this is done in one pan) inside the Heichel and sprinkle "the blood of the ox and the blood of the goat" on the inside altar. The fact that the posuk refers to the blood of the goat, even though it is mixed with the blood of the ox, which is greater in substance, shows that the minority retains its separate identity and is therefore not botel. For otherwise, there would be no separate identity of the blood of the goat. The blood of the ox and the blood of the goat are min b’mino (both blood). From here, Rabbi Yehudah learns to all the Torah that min b'mino is not botel even in a thousand.

It may be said, according to the Ran, that the Rabbis will explain that the din of min b'mino which is not botel (such as the blood of the goat) is dependant on two factors:

1) The nature of the two minim will be the same (however we define min, by name, taste or appearance). That is they will have the same name, the same taste or the same appearance.

2) Their din being the same.

Thus, in the case of the two bloods, not only do they have the same name, and appearance (blood). They also have identical dinim, in all respects. They are both osser, in the same amounts, in the same circumstances, with all the same details. Therefore, we say in such a case min b'mino is not botel. However, in the case of mixtures of heter and issur, for example treife meat with Kosher meat, although the two substances have the same name  (meat) and the same taste, they,however, have different dinim. One is muter and the other is osser. In such a case, we do not say min b'mino is not botel , but on the contrary, we learn that the issur will be botel in a rov of heter.

The Ran uses this understanding to explain the rule of dsyl"m. It is true that at the moment that we have to make a judgement, the mixture contains issur and heter and therefore the rules of bitel apply according to the Rabbis. Since however, the issur will become heter, then in a case of min b’mino, so far as name, taste or appearance is concerned, the mixture will be completely min b'mino, having the same name taste and appearance, and also having the same din of heter. At that time, therefore, the din of bitel will not apply. On the contrary, the din we learn from the blood, min b'mino where the name and the taste and the din are the same, will apply and there is no bitul. This understanding (lomdus) is the basis for the rule of the Rabbis that dsyl"m is never botel.