Musar zu Schemot 18:4

What is meant is that the entire Torah, all its laws and their spiritual impact, are the very soul of Israel, as I have explained at length on numerous occasions. This enables us to explain the various Midrashim on the paragraph. We read in Bamidbar Rabbah 19,7, that when Moses ascended to heaven he heard the voice of G–d expounding the law of the red heifer, quoting: "My son Eliezer says that the heifer has to be two years old, (the expression פרה, cow, means a minimum age of two years), whereas when the Torah uses the term עגלה, calf, this refers to a one year old animal." When Moses heard this, he said: "how is it possible that You, O G–d who owns heaven and earth, are sitting expounding Your Torah and quoting a human being as proof that Torah has been interpreted correctly!" G–d answered that at some point in the future a righteous person would arise who would commence his discourse on this portion by citing these words and his name would be Rabbi Eliezer, etc." Moses thereupon expressed the wish that the person in question should be one of his descendants. G–d immediately told him that his wish had been granted. This is the meaning of Exodus 18,4: "and the name of the one was Eliezer," when describing Moses' other son. The name of this special "son" of whom the Torah writes "האחד" is the Rabbi Eliezer whom G–d had quoted. He is the one who discoursed on the law of the red heifer.

I have explained why Moses named Gershon as he did, and why his second son Eliezer is not mentioned until Exodus 18,4, long after the Exodus. Only at that point does the Torah give the reason he had been called Eliezer, since G–d had already saved Moses from the clutches of Pharaoh.

When he had said "the G–d of my father has come to my assistance" (Exodus 18,4), he implied that even his father Amram had become so refined and purified that he qualified as an איש האלוקים. The words "He has saved me from the sword of Pharaoh," in addition to "the G–d of my father came to my help," mean that just as one grants additional wisdom to those who already possess a degree of wisdom, so Moses, seeing he had already possessed basic qualifications, advanced to even higher rank both spiritually and physically, so that even Pharaoh's sword could not harm his neck anymore than it could harm a pillar of marble. This is a hint that marble is incapable of becoming ritually impure. When the four sages entered the פרדס, i.e. the study of esoterics, מעשה מרכבה, Rabbi Akiva warned his colleagues "when you see pure marble, do not say "water, water!" (Chagigah 14)