John Calvin Commentary Exodus 22:6

"If fire break out, and catch in thorns, so that the shocks of grain, or the standing grain, or the field are consumed; he that kindled the fire shall surely make restitution." — Exodus 22:6 (ASV)

If fire break out and catch in thorns. This injury is somewhat different from the previous one, for the one who kindles the fire is commanded to make restitution for the damage he caused, even if there was no deliberate intention to do harm.

An incendiary who maliciously destroyed either a grain field or a vineyard was to be punished far more severely; here, however, mere carelessness is punished.

Although no specific mention is made of a house or barn, the law still includes all similar cases, requiring compensation from the one who kindled a fire even in an open field.

It might seem that such a person would be blameless, because he could not foresee that the fire would ignite the thorns. Yet, so that everyone would take as much care of another's property as of their own, God commands him to suffer the penalty for his careless or foolish negligence.