I am teaching a high school class about the Mitzva of Pru U’rvu. What is the Mitzvah M’doiraisa and what is the Mitzva M’derabanan? Please include which Pasuk we learn this from.
The Mitzvah D’oiraisa applies to having two children, one boy and one girl. The primary Passuk is in Parashas Noach 9:7.
It is also mentioned elsewhere in the Torah, Noach 9:1. Bereishis 1: 22. 1:28.
For an analysis of the multiple sources, see Sefer Hasichos 5751 I pp.75-88. See also Likkutei Sichos 30 p. 28 fns. 41-45.
The Mitzvah D’rabonon involves having children so long one has the ability, even after fulfilling the Mitzvah D’oiraisa. This in sourced in Koheles 11:6.
This Mitzvah is considered more than a regular D’rabanan; it is categorized often as Midivrei Kabala, meaning stemming from the Nevi’im. Some authorities consider it as a potential D’oiraisa, Sefeika D’oiraisa, as there is a possibility that the Mitzvah might not be fulfilled properly if one doesn’t continue to have children (we don’t know whether the original children will survive).
The Rebbe understands the position of the Rabmam that the Chazal’s Mitzvah isn’t a new Mitzvah, but rather an interpretation and extension of the Mitzvah D’oiraisa, possibly even fulfilling the D’oiraisa. See at length Likkutei Sichos 30 pgs. 24-29. See particularly p. 27 fn. 34.