On the illicit sexual relationships bring explicitly listed in the Torah - even that list leaves out some prohibited relationships. Plus, a statement by the mishnah on the essence of the Torah that the Gemara understands to be both explicit and derived details of halakhah. Also, chapter 2: Limiting the public study or discourse on certain topics, from the sexually illicit relationships to Ma'aseh Merkavah, and the metaphysics of the Divine. With some caveats, of course. Plus, other topics to avoid.