The Rambam writes:1 “Although a person may have already fulfilled the commandment to ‘be fruitful and multiply,’ he is commanded by the Sages not to neglect being fruitful and multiplying as long as he has the strength. For whoever adds a Jewish soul is considered as if he had built an entire world.”

“Be fruitful and multiply” is mentioned in the Torah portion of Bereishis, as well as in the portion of Noach.2 In the simple context of the verses,3 the expression in Bereishis is a blessing, while “be fruitful and multiply” in Noach is a command.

However, there is another reason why the source for the Rambam’ s statement that “whoever adds a Jewish soul is considered as if he had built an entire world” is derived specifically from the portion of Noach :

“Be fruitful and multiply” was told to Adam in Bereishis. Since this was said before he had children, it follows that it involved his fulfillment of the exhortation to “be fruitful and multiply” by fathering a son and a daughter.

In the Torah portion of Noach , however, “be fruitful and multiply and fill the earth” was said to both Noach and his children. This suggests that adding additional souls after one has fulfilled the commandment also falls within the parameters of being fruitful and multiplying; it is considered as if the person had “built an entire world.”

The above applies not only to physical children, but also to spiritual children, one’s Torah disciples, for “whoever teaches his friend’s child Torah is considered as if he bore him.”4 And in a more general sense it refers to any Jew that one draws closer to Judaism, to Torah and its commandments.

With regard to bearing spiritual children, there is absolutely no difference between one’s youthful years and one’s old age. As the Gemara states:5 “If one studied Torah in his youth, he should study Torah in his old age; if he had disciples in his youth, he should have disciples in his old age, for the verse states:6 [‘In the morning sow your seed], and do not withhold your hand in the evening.’ ”

Thus, just as the obligation of personal Torah study extends to one’s later years,7 so too with regard to the positive commandment8 of teaching Torah.

After a person has toiled to affect others during his youth and middle years, he may think that, having reached old age, he can rest from his labors. The verse therefore tells us “do not withhold your hand in the evening.” For every beneficial effect that one has on the spirit of a fellow Jew “builds worlds.”

On a deeper level, the explanation is as follows. The ultimate purpose of creation is to transform this world into a dwelling place for G‑d.9 This is the inner meaning of the phrase “building a world.” Left to its own devices, the world tends to conceal G‑dliness. A “new world” must therefore be built, one that reveals rather than conceals G‑dliness.

This change within the world is brought about through a change within the Jewish people: As long as there are Jews whose Judaism is in a state of concealment, i.e., whose actions do not reflect the fact that they are descendants of the Patriarchs and Matriarchs, then the G‑dliness that pervades the world is also in a state of concealment.

By striving to reveal each and every Jew’s connection to G‑d, Torah and mitzvos, it is considered as if one “built an entire world.”

Based on Likkutei Sichos Vol. XXX, pp. 24-30.