How much control to rabbis have over halakhah? How does machloqes work? The Ran explores this question and its relationship to their power to set the calendar, and why declaring the month is the first mitzvah in the Torah.

Then he discusses how halakhah relates to metaphysical existence, and how can we establish the truth of a non-physical claims – the soul, afterlife, etc… And thus, he claims that living according to halakhah is spiritually beneficial with possible physical outcomes.

The text is available with footnotes at https://www.aishdas.org/kloiz/Derashos-haRan-derashah-3.pdf (starting at ¶ 23).

Or on Sefaria, beginning at https://www.sefaria.org/Derashot_HaRan.3.23

Derashos haRan is an explanation of the fundamentals of the Torah as explicated from texts in the Chumash. In the third derashah, the Ran discusses Moshe, Aharon, the nature of nevu’ah, Moshe’s special kind of nevu’ah,, the urim vetumim, and the first mitzvos given to the Jewish People — counting months from Nissan and of Pesach.