In an article for Nitzavim-Vayeilech 5773, Rabbi Cherki explains the different roles of the oral and written Torah and the responsibilities of the leaders.
The words of Pirkei Avot (The Ethics of the Fathers) are well known – that Moshe received the Torah at Sinai and passed it on to Joshua (Avot 1:1). However, in the Talmud another tradition appears:
"We have been taught: How was the 'Mishna' organized? Moses learned straight from G-d. Aaron then came in and Moses taught him the passage. Aaron then moved over and sat on Moses' right side. His sons came in and Moses taught them. When the sons left, Elazar sat on Moses' right side, and Itamar sat to the left of Aaron. The elders came in, and Moses taught them. The elders then left and the entire nation came in and Moses taught them. Thus, Aaron heard the passage four times, his sons three times, the elders twice, and the rest of the nation once. Then Moses would leave and Aaron would review what he had learned. Aaron left, and his sons taught the people. They left, and the elders taught the people. Thus, everybody heard the material four times." [Eiruvin 54b].
According to this description, the Torah was handed down by the priests and the elders, not by Yehoshua!
A reasonable answer to this dilemma is to differentiate between two types of Torah – the written and the oral Torah. The purpose of the oral Torah, which in the above passage from the Talmud is called "Mishna," is to teach us the practical aspects of how we are meant to act. This is studied using the techniques of deriving the laws from the Torah, leading to the rules of the halacha, and not by a simple reading of the text. Such study requires a stable institution which can decide questions which are in doubt, a function that is performed by the priests and the elders. The written Torah, on the other hand, teaches the ethical values at the foundation of the Torah, and this can be derived directly from the text, even if it is not directly related to the halacha. An example is the law pertaining to a person who injures somebody else's eye – the written Torah indicates what he deserves in principal, to lose his own eye, while the oral Torah teaches us the practical laws of payment for damages.
The person in charge of passing on the ethical aspects of the Torah is the king, who reads the Torah "so that he will learn to fear his G-d" [Deuteronomy 17:19], and who in practice follows the written Torah in guiding the nation (see the Natziv, Deuteronomy 24:16).
Source: "AS SHABBAT APPROACHES" – a biweekly column in Shabbat B'Shabbato, NItzavim-Vayeilech 5773, Volume 1489. (Zomet Institute) See: www.zomet.org.il/eng