The Torah instructs that if a master takes an additional wife, "he shall not diminish" what he owes to the first wife (Exodus 21:10). Rabbi Yoshiyah raises an important question about who exactly is protected by this verse. Does the prohibition against diminishing food, clothing, and conjugal time apply to the new wife (the Jewess he is now marrying), or to the previous wife (the Hebrew maid-servant he already took)?
At first reading, the verse seems to protect the new wife. "If another he take for himself" could be read as: even though he takes another, he must provide for her fully from the start. The protection would attach to the woman being added to the household.
Rabbi Yoshiyah rejects this reading. The Torah says "he shall not diminish," and the concept of diminution only makes sense in relation to someone who has already been receiving something. You cannot diminish what has never been given. A new wife has not yet received food, clothing, or conjugal time from this man. There is nothing to diminish.
The Hebrew maid-servant, by contrast, has already been receiving these things. She was taken as a wife first. She has an established entitlement. When the master takes a second wife, the danger is that he will reduce what he provides to the first. The Torah anticipates this exact scenario and blocks it. "He shall not diminish" — the existing wife's rights remain intact regardless of what new arrangements the husband makes.
This ruling protects the most vulnerable person in the equation: the woman who was a maid-servant before becoming a wife. Her prior status does not reduce her current rights. Once married, she is entitled to the same undiminished provision as any other wife, and the arrival of a new wife cannot erode what she already has.