The Mekhilta records Rabbi Yishmael's ruling on which types of dough qualify for the matzah obligation on Passover — and the answer is far more restrictive than one might expect.
The previous discussion established that certain doughs are excluded from being considered matzah. But other types of enriched or processed doughs might seem acceptable: spongy dough (made with extra liquid), honey dough, pasty dough, pan-cakes, and pressed-cake. These are all made from the five permitted grains. They are all unleavened. Why would they not count?
Rabbi Yishmael points to (Deuteronomy 16:3), which calls the matzah of Passover "bread of affliction" — lechem oni. This phrase is not merely descriptive. It is definitional. The matzah that fulfills the commandment must be bread that reflects poverty and hardship. It must be plain, simple, and unadorned.
Spongy dough, honey dough, and the rest are too rich, too elaborate, too comfortable to qualify as "bread of affliction." They may be unleavened, but they are not humble. The commandment requires not just the absence of leaven but the presence of austerity. Matzah must taste like what it represents: the bread of slaves who fled Egypt with no time to let their dough rise and no luxury to enrich it.
Rabbi Yishmael's ruling ensures that the physical experience of eating matzah on Passover mirrors the historical experience it commemorates. The bread must be as bare as the moment it recalls.