“Anything we prohibit or practice today is only because of the commandments which G-d gave to Moshe Rabbeinu... We do not undergo circumcision just because Avraham Avinu circumcised himself and the members of his household, but rather because G-d commanded us through Moshe to circumcise our sons just as Avraham circumcised his sons (Rambam, Peirush Ha-Mishnayot, Chullin). The Torah was given at Sinai, and Jewish law was established then. Whatever our forefathers did, they did of their own volition and not because they were given a Torah mitzva.
As a result, even though G-d said to Avraham, “Your name shall be Avraham” (Bereishit 17:5) and our Sages derive from this that anyone who calls Avraham by his former name Avram is transgressing a positive commandment, such a mitzva is not included in the list of the 248 positive commandments. This is because Avraham’s story took place before the Torah was given at Sinai.
This principle, however, presents us with numerous difficulties. How is it that our Sages derive that one must be quick to perform a mitzva from the episode of Avraham arising early in the morning to fulfill the directive of G-d to sacrifice his son Yitzchak? How did our Sages learn from Lavan that we do not mingle semachot? (See the first essay in Parshat Vayetze.) How could our Sages derive the requirement of using a knife to slaughter an animal from the story of the sacrifice of Yitzchak, where the Torah says, “And he took the knife to sacrifice his son” (Bereishit 22:10)? There are many more examples.
A number of solutions have been proposed:
1. We do not actually derive mitzvot from stories about our forefathers. We do, however, derive details of how to fulfill them.
2. The only types of laws we derive from pre-Sinaitic times are those that are logical and have clear reasons behind them. We do not derive laws which are simply divine decrees (gezeirat ha-katuv) from this material.
3. If we have no other way to derive a law, and it does not appear among the laws given at or after Sinai, we may derive the law from material that appears before the giving of the Torah.
4. We derive the law from pre-Sinaitic sources only in cases where we can explain why this specific mitzva went into effect even before the giving of the Torah.
5. We can use pre-Sinaitic material to clarify words and other details of laws given at Sinai.