Commentary for Bava Kamma 11:8

What about Pit in that there is no other force involved with it? The Gemara rejects the original contention that a stone, knife and load that were left where the wind could blow them away, which then came to rest in a public domain and damaged there, could be derived from bor. The Gemara refutes the comparison to bor by arguing that שכן אין כח אחר מעורב בו the creation of a bor comes about solely through the efforts of the bor digger whereas the stone, knife and load must combine with another power, the wind, to become a damager. There is another refutation that the Gemara might have used to show the strength of bor as compared to the weak point of one’s stone, knife or load: שכן מעשיו גרמו לו, that his own action caused the damage, which is true of bor but not of the stone, knife or load which were assisted by the wind. The Gemara later (on this page) uses this refutation. Why does the Gemara sometimes use one description of bor as a refutation and sometimes another? Tosafot addresses this issue.

למאי דפריך ¹It seems that the proper text is כדפרישית as we explained, not כדפריך -as was asked.דלא חשיב אש מעשיו גרמו לו לפי שהרוח מסייע לו ה"מ למפרך הכא מה לבור שכן מעשיו גרמו לו תאמר באבנו וסכינו

According to what we explained (5b Tosafot כי שדית) that aish cannot be described as “his action causing the damage” because the wind assists the fire and the damager is therefore not totally a result of his own actions, the Gemara could have refuted the derivation from bor by saying; how can you compare the stone, knife and load to bor where his own action causes the damage? Is it logical to say that the same rule should apply to one’s stone, knife and load where it is not only his action that caused the damage, but the wind as well assisted in the creation of the damager? Why does the Gemara choose to refute the derivation from bor with the argument that the stone, knife and load had to combine with another power?
Tosafot is now going to tell us that the less avos that can be used to contradict the refutation, the better the refutation. If there are many avos that show that a refutation is not true, then initially the refutation is not very strong.
But because the refutation of the comparison to bor, that bor is not combined with any other power is a more effective refutation, the Gemara uses this refutation. Why is this refutation more effective? Because one cannot rejoin this refutation by presenting shor as proof that this factor does not preclude the status of a damager. Shor definitely does not have the weakness that it is combined with another power. It does have the weakness that it is not one’s action that caused the damage; the action of the shor is not one’s own action. Shor could be presented as proof that the refutation, it was not caused by his own action is inaccurate, but it cannot be presented as proof that the refutation it is combined with another power is inaccurate, because shor does not combine with another power. Only aish alone can be presented as proof that combining with another power does not preclude designation as a damager. Since only aish can be used as a rejoinder to the refutation, the stone, knife and load combine with another power, the Gemara chooses that refutation, as opposed to the refutation, a bor is his own action, which could have been refuted by shor as well as aish.
In the Gemara’s next answer of what is learned from the common characteristic the Gemara discusses a stone that was left in a public domain but did not damage at the location where the owner left it. It was rolled about in the public domain and damaged after it landed. There, the Gemara does use the refutation; that a bor is stronger because his own action caused the damage as opposed to the stone that was rolled about which caused damage only after someone else rolled it to its new location.
Why is that refutation of the comparison to bor used there and not the refutation that bor is not combined with another power as opposed to the stone which was kicked about that needs to combine with another power? And later by the bor that is rolled about by people or animals that kick the stone, the Gemara refutes the comparison to bor with the argument; how can you compare the kicked stone to bor? Bor has the quality that his own action caused the damage, which the kicked stone does not. It was the one who rolled it to its final location whose action caused the damage. Why does the Gemara use the weaker refutation that can be disproved by shor and aish and not the stronger refutation, that another power combined with his own which can only be disproved by aish? For it is not possible there to refute the comparison to bor with the argument that the bor is not combined with another power as opposed to the rolled stone which combines with another power, for how would we say at the conclusion of the argument; can you say the same for a bor that is rolled about which combines with another power that it is also liable? That argument would imply that the stone that was rolled about only becomes a bor by combining with another power. But that is not so, because it initially becomes a bor without combining with any other power when it was placed in the public domain. At that point it was a bor without combining with any other power, later when it damaged after it was kicked about, the damage was not the result of his action. It was the result of the action of those who kicked the stone to its new location.