Rebbi (Rabbi Yehudah HaNasi) noticed the same numerical tension between two biblical verses about the duration of Israel's time in Egypt. One says "they shall serve them and they shall afflict them four hundred years," while another promises "and the fourth generation will return here" (Genesis 15:16). Four hundred years spans far more than four generations. How can both be true?
Rebbi's answer is stunning in its theological implications. God gave Israel two possible timelines for redemption — and the choice between them depended entirely on Israel's behavior.
If the Israelites repented, God would redeem them by generations. Four generations — Abraham, Isaac, Jacob, and the tribes — and freedom would come. Repentance compresses time. A nation that turns back to God does not need to serve out a full sentence.
If they did not repent, God would redeem them by years. The full four hundred would need to elapse, every last one, before liberation arrived.
This teaching reveals a remarkable principle embedded in the Mekhilta's theology: divine prophecy is not a fixed sentence but a conditional framework. God built flexibility into the covenant from the very beginning. The same promise contained both a short path and a long one, and the key that determined which path Israel walked was repentance — teshuvah (repentance). The rabbis saw this pattern repeating throughout Jewish history: exile is never permanent, but its duration depends on whether the people turn back to God sooner or later.