Birchas hatorah from the Torah?

Question

I've heard many times that saying birchas hatorah is most likely from the torah (as apposed to mi-dirabanan). How could we not know? If it's from the torah, then it's from the torah!

Answer 1

Not necessarily. There are indeed various halachos where there is a dispute whether they are Biblical or Rabbinic obligations. One example that I can think of, offhand, is tevilas kelim (immersing dishes), about which Pri Chadash (Yoreh De'ah 120:1) cites variant opinions as to its source.

How did such a dispute develop? Same as any other machlokes. Details of halachah have been forgotten at various times in history (as far back as after Moshe's passing - Temurah 15b ff), and in trying to reconstruct them, different Rabbis will have different opinions. (There is a great book, The Dynamics of Dispute: The Making of Machlokess in Talmudic Times, by Zvi Lampel (Judaica Press, 1991), that discusses this topic from many different angles.)

Answer 2

The Gemarah in Sotah brings a Posuk as its source and many Rishonim learn it as an actual Torah source but Rambam maintains it is just an asmachtah, and really it is just like all other Birchos Hamitzvos. He is a daas yahid on this issue. It seems that the exact tradition whether it was midioraisa or not was not explicitly taught and the Rishonim on each side have their reasoning as to why they hold what they do. There are many instances of lost Mesorah which results in a later argument.