Ami Hertz Disproofs - Implicit proofs (Duran)

From Ami's  Disproofs series:

The purple text is from Rabbi Student,  the response in regular text is from Ami Hertz.

Implicit Proofs 13. R. Shimon ben Tzemach Duran points out that the Torah tells us that Jethro advised Moses to appoint judges. Jethro then told Moses [Ex. 18:20] "Enjoin upon them the laws and the teachings, and make known to them the way they are to go and the practices they are to follow." What does that mean? If the written law is all that was given, then there is nothing more for Moses to instruct these judges. What is Moses supposed to tell them, if not the oral law [Rashbatz, ibid.]?

1. Here is the full passage from Exodus 18:

Next day, Moses sat as magistrate among the people, while the people stood about Moses from morning until evening. But when Moses' father-in-law [Jethro] saw how much he had to do for the people, he said, "What is this thing that you are doing to the people? Why do you act alone, while all the people stand about you from morning until evening?" Moses replied to his father-in-law, "It is because the people come to me to inquire of God. When they have a dispute, it comes before me, and I decide between one person and another, and I make known the laws and teachings of God." But Moses' father-in-law said to him, "The thing you are doing is not right; you will surely wear yourself out, and these people as well. For the task is too heavy for you; you cannot do it alone. Now listen to me. I will give you counsel, and God be with you! You represent the people before God: you bring the disputes before God, and enjoin upon them the laws and the teachings, and make known to them the way they are to go and the practices they are to follow. You shall also seek out from among all the people capable men who fear God, trustworthy men who spurn ill-gotten gain. Set these over them as chiefs of thousands, hundreds, fifties, and tens, and let them judge the people at all times. Have them bring every major dispute to you, but let them decide every minor dispute themselves. Make it easier for yourself by letting them share the burden with you. If you do this--and God so commands you--you will be able to bear up; and all these people too will go home unwearied."

At the time, Moses was the only judge over the people. He had to work as a judge morning to evening. This was very tiresome, and could not do anything else. Jethro told Moses to appoint judges over the people. That way, Moses would only preside over the major cases; the minor cases would be resolved by the lower judges.
When Student mentions Exodus 18:20, he says that Jethro told Moses to teach the newly appointed judges. But that's not what Jethro said. Jethro was simply stating that Moses was the prophet through whom God was in the process of revealing His Instruction to the people. The verse is not a command and is not directed at the judges -- it is a statement that talks about all the people. Notice that Jethro mentions judges for the first time only in Ex 18:21.
2. Consider a typical math class. All the math is already in the textbook. Very often, the textbooks have much more information than is taught in class. There is no hidden "oral math" that is only passed from teacher to student orally. Yet, most people still need a math teacher to learn. It is possible to learn just by reading a book, and some people do learn that way. But the usual method of learning is by having both a textbook and a teacher.
3. Nehemiah 8 describes how Ezra read the Law of Moses in front of the people:

All the people gathered themselves together as one man into the broad place that was before the Water Gate; and they spoke unto Ezra the scribe to bring the book of the Law of Moses, which YHWH had commanded to Israel. And Ezra the priest brought the Law before the congregation, both men and women, and all that could hear with understanding, upon the first day of the seventh month. And he read therein before the broad place that was before the Water Gate from early morning until midday, in the presence of the men and the women, and of those that could understand; and the ears of all the people were attentive unto the book of the Law.

(If memory serves me right, while Ezra was reading, the priests were among the people teaching. Can't find that passage at the moment.)
It is the same principle. If the Law was already in a book, why did the people need Ezra? They could have just read to themselves (those who could read). They needed Ezra because people learn better with a teacher, even if all the material is already written.
4. If the written law is all that was given... At the point of Exodus 18:20, the (Written) Law was not yet given in full. For example, the famous Ten Commandments are only given in Exodus 20 for the first time. Since Moses was in the process of receiving the Law, and it was not yet completely written down, it was totally appropriate for him to instruct all the people in it, including the judges.
Moses says that "I make known the laws and teachings of God". This means that:

• He was an expert in the Law. Other people knew the Law generally, but they were not experts as they had to work at their own jobs. And,
  
• he was in the process of receiving the Law from God. As he was doing so, he made it known, and wrote it down.