In this week’s parshah the Torah commands us in the first mitzvah: pru u’revu – be fruitful and multiply. We rule in accordance with Beis Hillel that one fulfills this mitzvah when he has fathered one boy and one girl.
The Rambam (Hilchos Ishus 15:2) writes that women are exempt from this mitzvah while a man first becomes obligated in this mitzvah when he is 17 years old. Once he turns 20 and has not yet fulfilled the mitzvah, he has transgressed and is mevatel an assei. The Rambam adds, however, that if he is busy toiling in Torah and fears that if he marries the yoke of responsibilities will disturb his learning, he may prolong getting married. The reason he may prolong getting married is because the general rule is osek b’mitzvah patur min hamitzvah – when one is performing one mitzvah he is exempt from performing another. The Rambam concludes that we can certainly apply this rule in this case since the mitzvah that we are discussing is learning Torah – the greatest mitzvah of all.
The Acharonim were bothered by the Rambam’s explanation of this halacha. The Gemara in Moed Kattan 9a says that for a mitzvah that cannot be performed by anyone else, one must stop learning and we may not apply the concept of osek b’mitzvah patur min hamitzvah to the mitzvah of learning Torah. How then can the Rambam say that one may prolong getting married and be mevatel the mitzvah of pru u’revu because of osek b’mitzvah patur min hamitzvah, when that does not apply to the mitzvah of learning Torah? To make the question even stronger the Rambam added that we could certainly apply this concept here since we are dealing with the greatest mitzvah, learning Torah. And yet the exact opposite is true: specifically by the mitzvah of learning Torah we cannot apply this concept.
The sefer, Ma’aseh Rokeach, says that when the Rambam said that one could prolong getting married, he meant that this is so until he is 20 years old. That way one is not mevatel the mitzvah. So in essence there is no bitul mitzvah occurring, and therefore one may apply osek b’mitzvah patur min hamitzvah since one will not be mevatel the mitzvah in this case.
Rabbi Elchanan Wasserman, Hy”d, in Kovetz Ha’arus Hosafos 1, suggests that since the Rambam is referring to delaying the time until one gets married and not that one will never marry, we may liken this to a scenario whereby there is a mitzvah that can be done by others (since he can perform the mitzvah later) and we may thus apply the rule of osek b’mitzvah patur min hamitzvah even to the mitzvah of learning Torah. Reb Elchanan continues by explaining that the Rambam said that we could certainly apply this concept here since we are dealing with the mitzvah of learning Torah.
The reason why, in this scenario, this concept is more applicable to the mitzvah of learning Torah, even though we generally do not apply it to the mitzvah of learning Torah at all, is as follows: the reason why one must stop learning in order to perform a mitzvah that cannot be performed by anyone else is not because the mitzvah of learning Torah is inferior to all the other mitzvos, for it is indeed the greatest mitzvah of all. Rather, it is because when one must take care of his necessities (e.g., work for a living) there is no mitzvah of learning Torah. One is only obligated to learn Torah when he is free of his other obligations. When one is obligated to perform a mitzvah that cannot be performed by anyone else, the situation is no different and the obligation to learn is voided. However, if it is a mitzvah that can be performed by another person, or if he can perform this mitzvah at a later time, the obligation to learn Torah remains. Since he is obligated to learn we apply the concept of osek b’mitzvah patur min hamitzvah, and since the mitzvah of learning Torah is the greatest mitzvah of all we certainly apply the concept in this scenario.
In the following halacha the Rambam writes that one who never marries due to his sole desire to learn Torah, and always toils in it (like Ben Azai), has not transgressed. Reb Elchanan explains that even though in this scenario one is entirely mevatel the mitzvah, he has not transgressed because this is considered an oneis.