Rabbi Yitzchak disagreed with Rabbi Yoshiyah's reading of (Exodus 13:3), "and chametz shall not be eaten." He argued that the passive phrasing was not needed to equate the feeder with the eater — that principle could be established through logical reasoning alone, without requiring a dedicated verse.
His argument used kal va-chomer, reasoning from the lesser to the greater case. Sheratzim — creeping creatures forbidden by the Torah — carry a less severe prohibition than chametz on Passover. Yet even with sheratzim, Jewish law already equates the person who feeds the forbidden item to another with the person who eats it. If this principle holds for the less stringent case, it certainly holds for the more stringent one.
Since the logical derivation already covers the feeder-equals-eater rule, the verse "chametz shall not be eaten" must be teaching something else. Rabbi Yitzchak concluded that it forbids deriving any benefit from chametz during Passover — not just eating, but selling, trading, or using it in any way.
This debate between Rabbi Yoshiyah and Rabbi Yitzchak became one of the foundational disagreements in Passover law. Later halakhic tradition largely followed Rabbi Yitzchak's stricter position, which is why observant Jews today do not merely avoid eating chametz but also destroy it, sell it, or remove it entirely from their possession before the holiday begins.