"And if stolen, it shall be stolen from him" — the Torah establishes that a paid guardian is liable when the entrusted animal is stolen. But the Mekhilta asks: what about loss? If the animal simply wanders away and is lost, is the guardian equally liable?
The Mekhilta argues yes, through a logical comparison. Theft involves a lapse in the guardian's watching — he failed to prevent someone from taking the animal. Loss also involves a lapse in watching — he failed to prevent the animal from straying. The underlying negligence is the same in both cases: the guardian was not attentive enough.
Since the common element is the failure to watch properly, the legal consequence should also be the same. If the guardian is liable for theft (where someone actively took the animal), he should be equally liable for loss (where the animal simply disappeared due to inattention).
This derivation by analogy is characteristic of the Mekhilta's approach to guardian liability. The text does not merely list the specific scenarios addressed by Torah verses. It identifies the underlying principle behind each ruling and extends it to cover analogous situations. Theft and loss look different on the surface — one involves a criminal, the other does not — but at their core, both result from the same failure. The guardian's job is to watch. Whether the animal leaves by force or by accident, the guardian failed at watching. One failure, one liability.