The Torah is not in Heaven
After working seven years for Lavan for the right to marry his daughter Rachel, Yaakov was tricked. He thought he was being given Rachel as a bride, but after all was said and done he realized he had married Leah, Rachel’s sister. Lavan tried to justify his treachery, and concluded that Yaakov could marry Rachel as well once the week of celebrations ended. Yaakov did so, and thus was married to both sisters. Many authorities assume the Avos, the patriarchs, kept the entirety of the Torah before it was given. This is based on various allusions to such an idea. However, many struggle to reconcile this with the fact that the Torah explicitly prohibits a man from marrying two sisters. How then could Yaakov marry two sisters, which the Torah explains usually leads to strife?
To understand the answer to this question, the purpose behind the Avos keeping the Torah must be first clarified. If mitzvos are fulfilling the command of Hashem, what is accomplished by following something that wasn’t yet commanded? The Nefesh HaChaim explains that besides our universe, there are vast amounts of spiritual universes. They were created imperfect, and were further devastated by the sin of Adam and the Tree of Knowledge. Our job in this world is to fix them (popularly known as tikkun olam). This isn’t a physical fix; it’s a spiritual one. The way to accomplish these fixes is through the study of Torah and the performance of mitzvos.
The Avos were well aware of this fact, and this was their motivation in performing the mitzvos. They of course were not obligated in the laws of the Torah, otherwise they would have never transgressed any of them. However, they saw the direct spiritual effect each and every action of theirs created. They understood that performing certain actions as well as avoiding others, things which we know today as mitzvos, would affect a specific part of the spiritual geography in a tremendous way. They knew with their power of prophecy what their purpose in life was and what fixes they needed to make.
Therefore, when Yaakov saw the positive spiritual effect that would be created by marrying Rachel and Leah, two sisters, he couldn’t give up that opportunity. He saw that the twelve tribes of Israel would emerge from these marriages, and part of his mission in life was to bring this about. Through this act, the universe would be one step closer to perfection. He married them both despite the fact the Torah would later forbid marrying to sisters. Since the Torah hadn’t actually been given, Yaakov could use his best judgement to decide when the ends justified the means. This is one of the reasons the Torah wasn’t given to the Avos and was only given several generations later. If it had been already given, Yaakov and others could not have achieved their spiritual accomplishments.
However, once the Torah was given to the Jews, it is no longer in Heaven. Meaning, the Avos and others before the Torah was given could look to the stars, see the spiritual ramifications of their actions, and act accordingly. Now, even if doing a certain act could have some positive spiritual effect, if the Torah forbids it, there’s no exceptions. We can no longer look at the stars to determine what should be done. The reason is that no one today, even if they were hypothetically a prophet of the level of Moshe, can know the full ramifications of their actions. It may appear to create spiritual fixes in the immediate future, but there are ripple effects that are unknown. The Torah is telling us that these actions, which may seem beneficial, are really spiritually destructive in the long run.
There are allusions to this idea in the Torah. One is found in the parsha of the false prophet. The Torah warns us that a prophet may arise, perform miracles, and try to get the Jews to break the Torah. Despite the prophet’s seeming genuineness, their words are not to be heeded. Now, the fact that they can perform miracles shows us that they are a spiritually mighty person. Why then would they try to get others to break the Torah? It’s because with their prophetic powers, they can see the great spiritual fixes breaking certain mitzvos can accomplish. The Torah is emphasizing that despite this fact, this prophet should not be listened to. They don’t see the ramifications of what they are trying to accomplish. This is all because the holy Torah is above all human comprehension. How could its laws and applications be up to a person’s every whim? The Torah is no longer in Heaven; it is in our hands simply to learn and follow.
 Based on Nefesh HaChaim 1:21,22 and Minchas Asher Bereishis § 42
 According to Genesis 29:27 with Rashi
 Ibid verse 28
 Ibid verse 25
 Cf. Teshuvos Rema § 10 who says only Avraham kept the Torah, and Iggeres HaRambam to Rav Chisdai HaLevi that the Avos really didn’t keep the Torah. However, see note 10
 Kiddushin 4:14; Tanna D’Vei Eliyahu § 6; Genesis 26:5 with Rashi (based on Yoma 28b); Genesis 32:5 with Rashi; Bereishis Rabbah 64:4; Midrash Tanchuma Behar § 1; Midrash Tehillim 1:11
 See Ramban to Genesis 26:5 who says the Avos only kept the Torah in the land of Israel, Parshas Derachim and Maharsha to Yoma loc. cit. who say as converts to Judaism Leah and Rachel were no longer siblings, Gur Aryeh to 46:10 who says a similar idea (see there for other explanations), Teshuvos HaRashba I § 94 with the explanation of Teshuvos Radvaz § 696 for a more kabbalistic explanation (see note 11), Da’as Zekeinim to 37:35 who say that the Avos were selective about which mitzvos to observe (see note 11), Ohr HaChaim to 49:3 who says a prophet can at times of need nullify a mitzvah, and Igros Moshe Even HaEzer IV § 9 that without kiddushin there is no prohibition
 Leviticus 18:18
 Loc. cit., along with the most of Sha’ar Aleph
 Somehow Minchas Asher understood from the Nefesh HaChaim that the Avos didn’t physically perform actions, rather they made the same spiritual fixes that occur when we perform the mitzvos. He wants to use this explanation to justify the opinion of the Rambam that the Avos didn’t actually fulfill the Torah. Rambam meant in a physical sense, but the spiritual accomplishments were the same. He says that this is how Rabbeinu Avraham ben HaRambam understood in his commentary on the Torah, as well as the Oneg Yom Tov in his introduction. I don’t see how this fits in the words of the Nefesh HaChaim, and the Minchas Asher later retracts on this point and learns the Nefesh HaChaim as written above
 This could be the intention of the Rashba, Radvaz, and Da’as Zekeinim loc. cit. in their resolutions to this contradiction
 See Deuteronomy 30:12 and Bava Metzia 59b
 If so, why were the Avos allowed to do these actions which the Torah would eventually prohibit, since they also didn’t know the ultimate consequences of their actions? I heard in the name of Rav Yonasan David shlita that before the Torah was given, there wasn’t the same ripple effect as there is today with our mitzvos. Therefore, the Avos saw the direct results of their deeds, and could decide accordingly
 See Nefesh HaChaim for another from the story of Chizkiyahu HaMelech and Yeshaya found in Berachos 10a
 Deuteronomy 34:10