Ramchal makes the point that our sages “had a tradition that the ideas included in the (oral) tradition were hinted at in the Written Torah in different ways”, and he offered that they knew as well that “G-d wanted them to involve themselves in the process” of demonstrating this relationship. So they dug as deeply as they could until they found the oftentimes subtle ways that things given-over by word of mouth were alluded to in writing.
That’s why we often find discussions in the Talmud about “proofs” of a written tradition for an idea transmitted only orally. Now, there were times, as Ramchal puts it, when “on the surface, the proofs didn’t seem to fit very well” into the literal meaning of the text. So the sages offered that while some things were indeed left unsaid in the text, others were hinted at — albeit in a rather indirect way. They termed such hints “Asmachtot” (literally, “things to lean upon”) meaning to say that they could be depended upon by the faithful as representations of the tradition.
Sometimes Asmachtot were also cited by the sages to allude to laws that they themselves instituted, which seems absurd. But as they explained, since everything they taught was rooted in the Written Torah, it only stands to reason that their enactments would be cited there on some level. As Ramchal says, it was as if G-d — who knows everything beforehand , and has set everything in motion from the first through His instructions in the Torah — did indeed refer to the rabbinic enactments in His Written Torah indirectly beforehand.
The sages explained in fact that there were four levels of interpreting the Torah, known as “PaRDeS”, which is an abbreviation for the terms P’eshat (referring to the literal meaning of the text), R’emez (the allegorical meaning), D’rush (the homiletic meaning), and S’od (the mystical meaning). And since, as the great Kabbalist R’ Yitzchak Luria said, the process of finding proofs in the text for oral traditions falls under the category of Remez (Sha’ar HaGilgulim 17), it follows that the other methods of discussing the interplay between the Oral and Written traditions were part of the larger PaRDeS process.
That’s not to say that there weren’t differences of opinions among the sages about the authority for rules as time went on, for there were very many variations. But the Torah provided a procedure for resolving that, which is where practical Halacha comes into play.
Rabbi Yaakov Feldman has translated and commented upon “The Gates of Repentance”, “The Path of the Just”, and “The Duties of the Heart” (Jason Aronson Publishers). His works are available in bookstores and in various locations on the Web.