“Because of this the children of Israel, to this day, do not eat the thigh muscle that is on the socket of the hip, since Jacob’s hip socket was wrenched at the thigh muscle” (Breisheet 32:33).
The prohibition to eat gid hanasheh dates to Yaakov’s encounter with the mysterious “man” as, alone at night, he prepared to meet Eisav. It is the third of the three mitzvot that appear in sefer Breisheet, following on the heels of pru urevu and brit milah.
Having been given before Sinai, it stands to reason that the prohibition to eat gid hanashe is not part and parcel of the not-yet-given laws of kashrut. Rather, it is a separate stand-alone mitzvah and, based on its historical context, applicable to all animals. As Rabbi Yehuda notes (Chulin 100b), the gid hanasheh was prohibited from the time of Yaakov, a time when the concept of kosher and non-kosher animals did not yet exist; and hence, if the Torah prohibits the gid hanasheh, it must apply to all animals, kosher or not.
Nonetheless, the Sages disagree and limit the prohibition to kosher animals only. “They said to him: It was said at Sinai, but written at its place.” This retort of the Sages is, in the words of the Rambam (Commentary to Mishna 7:6), “a major principle of Judaism” to which we must pay extra attention. Mitzvot are obligatory because, and only because, they were given by G-d to Moshe at Har Sinai. The historical origins of the prohibition of gid hanasheh may stem from Yaakov’s encounter with the “man”, and that may be the reason the Torah prohibited the gid hanasheh in the first place. However, it was only at Sinai that the prohibition was affirmed and thus became obligatory.
Similarly, the Rambam notes that the obligation to have a brit milah is not because G-d commanded Abraham to do so, but rather because the mitzvah was given to Moshe at Sinai and is recorded at the beginning of parshat Tazria. Nor, the Rambam explains, is the prohibition of eating ever min hachai, a limb torn from a living animal, incumbent upon us because it is one of the “seven mitzvot [incumbent] on all descendants of Noach”, i.e., all of humanity, but rather because it, too, was commanded at Sinai—being derived from the verse, “However, be strong, do not eat the blood, for the blood is the soul; and you shall not eat the soul with the flesh” (Devarim 12:23). As Rashi explains, this verse teaches that as long as the “soul” is in the animal—i.e., it is still alive—we may not eat from the animal.
This last example is a fascinating one, as the prohibition of ever min hachai applies to non-Jews, who received no commandments at Sinai. In fact, the Rambam’s proof to his “Sinai only” theory is based on the teaching that “613 mitzvot were given to Moshe at Sinai” (Makkot 24a). It is almost as if, according to the Rambam, the opening sections of the Torah have legal applicability for non-Jews only!
It is exceedingly difficult to take the notion that the 613 mitzvot originated at Sinai literally. Our Sages note that the mitzvot of Shabbat, judgement and kivud av v’eim were given at Mara (Sanhedrin 56b), prior to their arrival at Sinai. And that is before we open up parshat Bo, where many of the mitzvot relating to Pesach are recorded. Were these all given again at Sinai and “written in its place”?
More difficult are the many mitzvot as a result of the events occurring after Sinai, including the laws of inheritance in response to the inquiry of the daughters of Tzelofchad, and the prohibition to accept (male) converts from the nation of Moav because they refused to give us bread and water in the desert. While it is theoretically possible to claim that these, too, were given at Sinai and “written in its place”, this does not appear to be the plain meaning of the text (nor would it explain why Moshe was “stumped” by the question of Tzelofchad’s daughters)—and clearly is not the view of Rav Yehuda. And if the Rambam might understand that Sinai can include post-Sinaitic commands, is it unreasonable to assume it can include those written beforehand by the Divine author?
Sinai is the fulcrum around which the Torah revolves, and is where G-d revealed Himself to the Jewish people; it is where the marriage of G-d and the Jewish people occurred. But mitzvot can emanate from anywhere and everywhere in the Torah. Taking the “613 mitzvoth were given to Moshe at Sinai” phrase even less literally, the BeHag, as we discussed in our last post, extends his list to include rabbinic mitzvot. It was at Sinai that we were commanded to listen to the Sages.
Interestingly, the Rambam gives the examples of ever min hachai, brit milah and the gid hanasheh as pre-Sinatic “mitzvot” that are mitzvot only because they were repeated “at Sinai”. He does not mention pru verevu, the first mitzvah mentioned in the Torah, nor any of other six mitzvot b’nai Noach incumbent on all humanity, leaving one to wonder why he mentions ever min hachai at all.
The Rambam (Introduction to Avot, Chapter 6) distinguishes between two types of mitzvot: the “rational mitzvot” that are “well-known to all people” and would be observed even had they not been commanded at Sinai, such as murder, theft, robbery, fraud, hurting someone who did you no harm, doing evil to those who were good to you, belittling father and mother”, and those mitzvot “that if not for the Torah, would not be wrong at all…[such as the prohibition of] meat and milk, wearing sha’atnez and sexual immorality”.
Having children, and the other six mitzvot b’nai Noach—the prohibition of murder, adultery, theft, idolatry, blasphemy, and the obligation to set up courts of law—are rational mitzvot for which a divine command was not needed. They may have been said at Sinai, but did not need Sinai to be said.
When all is said and done, it matters little if our responsibilities were given to us specifically at Sinai, elsewhere in the Torah, or even legislated by the rabbis. One who yearns to fulfill the Divine will attempt to observe them all.
 Interestingly, the mitzvah of pru verevu is first given to Adam and then again to Noach, as they were the fathers of civilization. Brit milah, establishing the special covenant between G-d and the Jewish people, is given to the first Jew, Avraham. The gid hanasheh, reflecting the danger of night and exile, is given to Yaakov, who represents the Jew in exile. Yitzchak, our passive patriarch who obediently listens and follows the path of his father, is not given a specific mitzvah, but is the passive partner in the mitzvah of brit milah.
 This verse clearly implies that even animals have a soul, something that most assume is unique to humans.
 Rashi, when quoting this teaching (Shemot 15:25) substitutes para adumah for kibud av v'eim. These mitzvot are polar opposites the former being the most natural of mitzvot and the latter being the "least rational" of all mitzvot. These two mitzvot are beautifully linked together by our Sages as we discussed here.
 That sexual sins are included in the category of those mitzvot that are not obvious and clear is most telling. Apparently, there is nothing inherently wrong with two consenting adults—be they male or female—engaging in sexual activity outside of marriage. It is wrong only because the Torah says so. Yet at the same time, clearly some sexual sins—for example, adultery—are inherently wrong, even abhorrent. Most significantly, the Torah introduces the laws of sexual ethics (Vayikra, chapter 18) by commanding that we observe these chukim and mishpatim, the rational and the not-so-obvious laws relating to sexual ethics.