The Gemara in Sanhedrin 71a tells us a few mitzvot in the Torah that will never happen:

First, a ben sorer u'moreh

בן סורר ומורה לא היה ולא עתיד להיות ולמה נכתב דרוש וקבל שכר

There has never been a stubborn and rebellious son and there will never be one in the future, as it is impossible to fulfill all the requirements that must be met in order to apply this halakha. And why, then, was the passage relating to a stubborn and rebellious son written in the Torah? So that you may expound upon new understandings of the Torah and receive reward for your learning, this being an aspect of the Torah that has only theoretical value.

The Gemara then adds that an idolatrous city will never happen:

עיר הנדחת לא היתה ולא עתידה להיות

There has never been an idolatrous city and there will never be one in the future

And finally the Gemara states that a house that was afflicted with leprosy will never happen:

בית המנוגע לא היה ולא עתיד להיות

There has never been a house afflicted with leprosy of the house and there will never be one in the future

I have a couple of questions:

  • Why would Hashem gives us mitzvot that would never happen?
  • To expound and receive a reward? Are we experts enough in all other areas of the Torah that we don't need to spend time on what we already have?
  • Why specifically these 3? Why aren't there more mitzvot mentioned that will likely never happen?
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    This question may be nearly impossible to answer. Consider questions 1 and 4, which assume that that Torah shouldn't have given these mitzvos and that the Torah should have given more such mitzvos. #2 and #3 are not clear. I would recommend sharpening the list and possibly eliminating #2-4 to solicit the best answers. – WAF Feb 6 '19 at 17:37
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    To teach us the moral lessons from these mitzvos (and from the fact that they will/should never happen). – רבות מחשבות Feb 6 '19 at 17:43
  • @רבותמחשבות but is the Torah a mussar book? Maybe stories about the Avos etc, but actual mitzvot whose true purpose is to serve as mussar? – alicht Feb 6 '19 at 17:46
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    Your first quote has an answer to question #1: "So that you may expound upon new understandings of the Torah and receive reward for your learning" – Daniel Feb 6 '19 at 17:49
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    I don't have a source right now, but Chabad Chassidut explains that Mitzvot are the Will of G-d. Regardless of whether we can fulfill the commandment practically, doesn't change the fact that G-d wants such and such an outcome if such and such a situation arises. The Rambam uses similar logic to explain how wiping out Amalek is one of the 613 mitzvot, even though the Rambam holds in order to be part of the 613 mitzvot it has to be eternal. Because if Amalek would continue to exist, we would be still be commanded to wipe them out – Menachem Feb 6 '19 at 18:49

In a shiur by Rav Zvi Sobolofsky on YUTorah called "Inyanei Sfeikos 12," Rav Sobolofsky discusses the
"בית המנוגע" (house of leprosy) and notes that while even though this is something the gemara says will never happen, there are nonetheless very important practical halachos that are learned from it.

For example:

  • Chazakah - The gemara on Chullin 10b tells us the principle of chazakah (דמעיקרא) is learned from houses of leprosy.

מנא הא מלתא דאמור רבנן אוקי מילתא אחזקיה אמר רבי שמואל בר נחמני אמר ר' יונתן אמר קרא (ויקרא יד, לח) ויצא הכהן מן הבית אל פתח הבית והסגיר את הבית שבעת ימים

From where is this matter that the Sages said: Establish the matter on its presumptive status derived? Rabbi Shmuel bar Naḥmani said that Rabbi Yonatan said that the verse states [with regard to leprosy of houses that after a priest views a leprous mark]: “And the priest shall emerge from the house to the entrance of the house, and quarantine the house seven days”

(see said gemara for the full discussion)

  • K'dei Achilas P'ras - This is a defined amount of time in halacha, approxiamtely 3-4 minutes (see: brachos). The source for k'dei achilas p'ras is ALSO derived from a house of leprosy; namely it's the amount of time required for one to become ritually impure when entering a house afflicted with leprosy. (see Succah 6a).

היה לבוש כליו וסנדליו ברגליו וטבעותיו באצבעותיו הוא טמא מיד והן טהורים עד שישהה בכדי אכילת פרס

If he was dressed in his clothes, and his sandals were on his feet, and his rings were on his fingers, he immediately becomes ritually impure, but they, the clothes, sandals, and rings, remain pure until he stays in the house long enough to eat half a loaf of bread.

Thus while a house with leprosy may not be practical, nonetheless the halachos learned for it are very much so- and presumably a Ben Sorer U'Moreh & an idolatrous city also have practical halachot/ concepts that are learned out from them.

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