Shevuas HaPikadon, Mishnah 1. This mishnah opens the discussion of the oath of the guardians: a person who watches over an object, and something happens to that object - under certain circumstances he is able to take an oath and thereby be considered exempt, and he does not have to pay.
"Arba'ah shomrim hen" - there are four types of guardians:
Shomer chinam - one who watches an object without payment.
Ha'sho'el - the borrower, who borrows the object for his own use.
Nosei sachar - a paid guardian, who receives payment for watching.
Ha'socher - one who rents the object.
In the last two categories, as we will see, the halachah is identical.
"Shomer chinam nishba al hakol" - an unpaid guardian takes an oath about everything:
A guardian who receives no payment takes an oath about anything that happened to the object: if it was lost, if it was stolen, if an unavoidable accident befell it, or if it died - in all of these he is able to take an oath and be exempt. And what does he swear to? That he was not negligent and did not fail in his watching, since even an unpaid guardian is liable for negligence.
The borrower:
The borrower stands at the opposite extreme. Since all the benefit is his and he receives the object for free, he must pay for everything - for anything that happens to it. However, if the animal died in the course of its normal work, this is "meisah machmas melachah" (it died because of the labor), and for that he is exempt. In other words, although as a rule the obligation to pay rests on him in every case, in this case he is considered exempt.
The paid guardian and the renter:
A paid guardian and a renter "nishba'in al ha'shevurah ve'al ha'shevuyah ve'al ha'meisah" - they take an oath about an animal that was broken, one that was captured, and one that died. These are three types of unavoidable accident, things that happen to a person against his will: the animal was broken, meaning one of its limbs was broken; or it was taken captive; or it died. These are matters beyond his control. For these items the paid guardian and the renter take an oath that they were not negligent, that they did not fail in their watching and allow this to happen, and that this is indeed what happened to the animal.
"U'meshalem es ha'aveidah ve'es ha'geneivah" - but if the object was lost or stolen, they must pay. For these they are responsible, since such things are considered far more within their ability to guard against.
In summary: There are four guardians - an unpaid guardian, a borrower, a paid guardian and a renter. An unpaid guardian takes an oath about everything and is exempt, provided that he was not negligent; the borrower pays for everything, and is exempt only in a case of "meisah machmas melachah"; while the paid guardian and the renter, whose law is the same, take an oath about an animal that was broken, captured or died, and pay for loss and theft.