Detailed Explanation

1. A Borrower derives absolute benefit from the borrowed property; receiving the  entitlement for sole usage of the article[Sha"ch - Choshen Mishpat 340:3] without having to pay a fee. Hence, the borrower assumes complete responsibility including damages due toa)negligence b) theft/loss c) general unforeseen accidental damages beyond his or her control with the exception of paying for damage that occurs simply as a result of using the article in its intended use. [Exodus 22:13].

By means of introduction, it is worthy to familiarize ourselves with the following two significant concepts.

2.       The burden of proof rests upon he/she  wishing to alter the current status quo [Maseches Bava Kama 46a] .

Though, as explained below, numerous considerations affect the classification of the current status quo.

For instance, consider the following two similar, yet starkly varying scenarios.

A.       "Defendant (A), do you remember that I lent you three-hundred dollars a month ago, while sitting in Pierre Pizza Parlor on Pratt Boulevard? Could you please pay me back?"

 "Quite frankly, Plaintiff (A) I do not remember if I you lent me anything."  

B.       "Defendant (B), do you remember that I lent you three hundred dollars a month ago, while sitting in Pierre Pizza Parlor on Pratt Boulevard? Could you please pay me back?"

"Quite frankly, Plaintiff (B) I do  remember that you indeed lent me the funds, but I do not remember if I paid you already."

A.       Status quo: Defendant (A) is debt free. To compel defendant (A) to pay, the burden of proof rests with Plaintiff (A)

B.       Status quo: Plaintiff (B) is due payment. To absolve himself from repaying the undisputed loan, the burden of proof rests with Defendant (B) otherwise, defendant (b) must pay [Sh"ach Choshen Mishpat 291: 44].

3. As noted above, a borrower is liable for unforeseen accidents but absolved from recompensing damages simply resulting from normal course of use.

Q. When the cause of damage is in question,[like in Snow Down!] upon whom does the onus of proof fall? Alternatively, to whom does the status quo favor; the lender or the borrower? Simply put, is a borrower innocent until proven guilty or guilty until proven innocent?

A. When does the borrower's liability begin, upon borrowing the article or upon its loss?

Assuming liability sets in from the onset of the bailment, the borrower is liable to return/pay for the object until he/she proves otherwise: guilty until proven innocent, or the status-quo favors the lender. Assuming liability sets in at the moment of loss, the borrower is only liable to pay for damages if the lender can prove that the damage was not a result of its normal course of use; the borrower is innocent until proven guilty; the status quo favors the borrower.

Persuasive and compelling support exists to substantiate both viewpoints.

As such, when the cause of damage is questionable [unforeseen accident/damage as a result of normal usage] , the lender would be unable to compel the borrower to pay without proving that normal course of usage did not cause the damage .

Conversely, if the borrower already paid, he/she would be unable to demand the money back without proving that normal course of usage caused the damage [Sh"ach Choshen Mishpat 291: 44]..

4.. The competent opinion of an mutually trusted professional or one trusted by Beit Din, is valid proof to ascertain the cause of the damage.

Application

Ronny borrowed Alan's truck to use in the snow. Ronny is absolved from damages resulting from using the truck in the intended manner for which he borrowed it. Ronny is liable for any other damages even due to unforeseen circumstances. As a borrower, he is required to return the truck to Alan/ compensate him for damages during the course of the bailment.

If the mutually trusted mechanic ascertains that the truck broke down due to the use in the snow, Ronny is not liable. Otherwise, he is liable to pay for the repair.

If however, the cause of the breakdown cannot be pinpointed, Alan cannot obligate Ronny to foot the bill. Yet, if Ronny already paid the bill, Ronny cannot compel Alan to reimburse him.