The Mekhilta raises one of the most characteristic questions in all of rabbinic literature: if a law can be logically deduced from another law, why does the Torah bother stating it explicitly? The case here involves a borrowed object. The Torah says: "If its owner were not with him, pay shall he pay" (Exodus 22:13). Then, in the very next verse: "If its owner were with him, he shall not pay" (Exodus 22:14).
The rabbis' question is razor-sharp. If the first verse already tells us that a borrower must pay when the owner is absent, then obviously the reverse must be true: when the owner is present, payment is not required. So why does the Torah spell out what we already know?
This seemingly simple question opens a window into one of the deepest principles of rabbinic interpretation. The rabbis held that no word in the Torah is superfluous. Every phrase, every repetition, every seemingly redundant statement must teach something new. If the Torah states what can already be inferred, it must be adding a legal nuance that pure logic would miss. Perhaps the explicit statement broadens the exemption, or narrows it, or applies it to a case the reader would not have imagined. The Mekhilta does not always resolve these questions neatly. Sometimes the question itself is the lesson: train your mind to notice what seems obvious, because in the Torah, nothing is merely obvious.