When two people join together to perform a prohibited act on, according to Rabbi Yehuda and Rabbi Shimon, neither of them will be held liable. If, however, one of them could have performed that act entirely on his own, there is an element of liability. The Gemara discusses this ruling on today’s daf.
We learned earlier that the Master said: In a case where this person is capable, and this person is incapable, and they performed it together, everyone agrees that he is liable. The Gemara seeks to clarify: Which of them is liable?
Rav Ḥisda said: The one who is capable of performing the act alone is liable, as if it was the one who is incapable of performing the act alone that was liable, what is he doing that would render him liable? His efforts are inadequate to perform the task.
Rav Hamnuna said to Rav Ḥisda: He is doing quite a bit, as he is assisting him.
He said to him: The assistance provided by one who assists another to perform a task that the other could have performed himself is insubstantial.
The Gemara’s question is difficult to understand. What reason could there be to deem the one who was incapable liable, and the one who is capable exempt? Tosafot restricts the Gemara’s question to whether or not the one who was incapable is as liable as one who assisted in the performance of the prohibition. The Rashba explains the Gemara differently. According to Rabbi Yehuda and Rabbi Shimon, in a situation where both are capable of performing the act, both are exempt if they perform it together. The reason that they are exempt is because neither one is expending his full effort in its performance. Consequently, in a situation where one is capable and one is not, the one who is capable does not expend his full effort, and therefore he is exempt. In contrast, the one who is incapable would be liable because he is expending his full effort. From the perspective of the one who is incapable, this is tantamount to a case in which both people are incapable and liable.