As we learned at the beginning of Massekhet Bava Kamma (2a), there are four avot nezikin, each with its own set of rules. Although the Gemara does not discuss the underlying reason for responsibility in most of these cases – apparently the reason that a person is responsible for the damage his property has caused is fairly intuitive – our Gemara does discuss why a person is responsible for damage caused by a fire that he set. Tosfot Rabbeinu Peretz explains that there is a need to explain esh (fire), since it is qualitatively different than the others. In esh there is no direct link between the damage and the person who is responsible for it; someone set the fire and then it was blown about by the wind, and only then did it cause damage.
Rabbi Yohanan teaches that, “Isho mishum hitzav – a person is responsible for his fire,in the same way that he is responsible for arrows that he shot.” Although the damage was not done by a direct encounter, nevertheless it is clear that the owner has set the flame on its course, just like someone who shoots an arrow from afar will be held accountable for its results. According to Reish Lakish, “Isho mishum mamono – a person is responsible for his fire, in the same way that he is responsible for damage done by any of his possessions.”
Rashi explains Reish Lakish’s position as positing that the fire belongs to him, and he is responsible in a similar fashion to damage done by anything else that he owns. According to this approach, a practical difference between Rabbi Yohanan and Reish Lakish would be a case where the fire did not belong to the man who set the fire, where Reish Lakish would, apparently, free him from responsibility.
Tosafot and other rishonim object to this conclusion and they explain isho mishum mamono as meaning that the damage was done by something that the man owned or was created by his actions – similar to his responsibility for a bor (a pit), and there is no real difference whether he actually owned it or not.