The Mekhilta examines a specific scenario in the laws governing borrowed property. If an animal passes from the domain of a lender to that of a borrower, even for a single moment, the rules of liability shift depending on one crucial factor: whether the owner is present.
The Torah establishes that when someone borrows an animal and the owner is "with" the borrower, meaning present at the time of the loan, the borrower is exempt from liability if the animal is injured or dies. But if the owner is not present, the borrower bears full responsibility for any harm that befalls the animal.
The Mekhilta's contribution here is the emphasis on the phrase "even for one moment." The transition from one domain to another need not be prolonged. The instant the animal crosses from the lender's possession to the borrower's, the legal framework activates. There is no grace period, no transitional zone. The moment of transfer is the moment liability is determined.
This ruling illustrates the precision of rabbinic civil law. The sages were not content with general principles. They wanted to know exactly when responsibility begins, down to the split second. The presence or absence of the owner at that precise moment determines everything. If the owner handed over the animal personally and remained nearby, the borrower is protected. If the animal arrived without the owner, the borrower assumes all risk. Jewish law treats property transactions with the same exactitude that it applies to ritual matters, because in the rabbinic worldview, justice between human beings is no less sacred than service to God.