Rabbi Yonathan disagrees with Rabbi Yoshiyah's reading of "he shall not diminish" (Exodus 21:10). Where Rabbi Yoshiyah understood the verse as protecting the Hebrew maid-servant (the first wife), Rabbi Yonathan argues that Scripture here speaks of a Jewess in general, meaning the verse establishes rights for all Jewish wives, not just former maid-servants.
Rabbi Yonathan anticipates an objection. If the verse speaks of a Jewess in general, perhaps it also speaks of the Hebrew maid-servant. Maybe the verse protects both categories simultaneously. He resolves this by pointing to the preceding verse: "according to the ordinance of the daughters shall he provide for her" (Exodus 21:9). That verse already addresses the maid-servant's rights. Her food, clothing, and conjugal time are guaranteed by the earlier passage.
Since the maid-servant's protection is already established in verse 9, verse 10's instruction "he shall not diminish" must address someone else. Otherwise, the Torah would be repeating itself unnecessarily. By process of elimination, "If another he take for himself" speaks of a Jewish wife in the broader sense. The verse extends the three-fold obligation beyond the specific case of the maid-servant to cover all marriages.
The practical difference between the two Sages' readings is significant. Under Rabbi Yoshiyah's interpretation, the verse protects a specific woman in a specific circumstance. Under Rabbi Yonathan's interpretation, it establishes a universal marital obligation. Every Jewish husband, regardless of whether a maid-servant was involved, must never diminish his wife's food, clothing, or conjugal rights when he takes an additional wife. Rabbi Yonathan transforms a narrow protection into a foundational principle of Jewish marriage law.