לע"נ מרת אסתר בת שמואל
ומרת חנה בת ר' יצחק יונה
Krisus 19b - Said Rav Nachman in the name of Shmuel: Unpurposed eating of chelev or [unpurposed] incestuous intercourse is subject [to an offering], because the offender has after all derived a benefit thereby; unpurposed labour on the Shabbos is exempt, because the Torah has forbidden only purposive work. Said Rava to Rav Nachman: Surely the case concerning [the circumcision of] boys is comparable to unpurposed action, and yet we have learnt regarding it: If there were two boys, one who was due to be circumcised on Shabbos and another who was due to be circumcised after the Shabbos, and a person in error circumcised on Shabbos the one who was due to be circumcised after Shabbos, R. Eliezer declares him liable to a sin-offering; R. Yehoshua holds: He is exempt. Now R. Yehoshua declares him exempt only because he maintains that for [a transgression committed in] error in the course of the [intended] performance of a commandment, even though the commandment was not in fact performed, one is exempt; if, however, one performed an unpurposed act which was not in the course of the performance of a commandment he would be liable even according to R. Yehoshua. — He replied to him: Leave the case concerning the [circumcision of] boys alone. Since [it is exceptional in that] one is liable although the wound is an act of damage; so too, for unpurposed wounding one is also liable.
Tosfos in Sanhedrin [62b] asks why Shmuel says that מתעסק is patur because it is not מלאכת מחשבת when we have a pasuk - אשר חטא בה - which excludes מתעסק. In their first answer Tosfos say as follows:
וי"ל דהכי קאמר בשבת פטור אע"פ שנהנה משום מלאכת מחשבת.
Apparently what Tosfos means is that when there is no הנאה we don't need to come to the reason of מלאכת מחשבת אסרה תורה because we can derive from the pasuk אשר חטא בה and exclude מתעסק. It is only when there is הנאה that we need to revert to the סברא of מלאכת מחשבת אסרה תורה. The reason is that when there is הנאה the פטור of מתעסק doesn't apply as Shmuel said that המתעסק בחלבים ועריות חייב שכן נהנה.
So now we are in a quandary. The gemara said that with respect to חבורה there is no din of מלאכת מחשבת and one is חייב even if it is מקלקל. But why - let the person be patur because of the pasuk אשר חטא בה which excludes מתעסק. As we saw from Tosfos - when there is no הנאה we can learn that מתעסק is patur from the pasuk.
Tosfos in a number of places [Sanhedrin 62, Shabbos 72b] say that they are two distinct, unrelated exemptions. The ptur of בה פרט למתעסק that applies to the whole Torah was only said regarding one who was מתעסק בהיתר i.e. he thought he was dealing with something permitted and it turns out that the very same item was forbidden. The ptur of מלאכת מחשבת that was said regarding איסורי שבת only refers to where he was dealing with two separate items. For example he wanted to cut a cucumber on the ground which he thought was detached but lo and behold he cut up a DIFFERENT cucumber which was attached to the ground. But when he was dealing with one item which he thought was hetter and it turns out it was issur, that is considered a מלאכת מחשבת because he did what he intended to do.
However, the Rambam [Shabbos 1/8] writes as follows:
Whenever a person intends to perform a forbidden labor, but instead [through his actions] causes the performance of another forbidden labor for which he had no intent, he is not liable, because his intent was not carried out.
What is implied? A person threw a stone or shot an arrow at a colleague or at an animal with the intent of slaying them. Should [the object that he propelled] uproot a tree in its progress and not kill [the intended victim], he is free of liability.
How much more so does this principle apply if one had the intent of performing a lesser transgression and one performed a more serious one. For example, a person intended to throw [a stone] into a carmelit, and instead, the stone passed into the public domain. He is not held liable. The same applies in other similar circumstances.
Should a person have the intent of performing a permitted act and instead perform another act [which is forbidden], he is not held liable. For example, should he intend to cut produce that was not attached to the ground, and instead cut produce attached to the ground, he is not held liable. The same applies in other similar situations.
The Maggid Mishna there references the machlokes rishonim about one who is מתעסק as he intended to remove detached produce and instead removed attached produce, does this mean that he made a mistake and removed a DIFFERENT attached produce or that the produce he intended to remove was not detached as he assumed but was attached. Writes the M.M. that the correct approach is that of the Ramban that in all instances he is מתעסק even when he intended to remove that very same item. It seems that the M.M. is learning this way according to the Rambam as well. And this makes sense because the Rambam gives no indication that he differentiates between because one item or two [as does Tosfos].
See also Hil. Shgagos [7/11] where the Rambam doesn't distinguish between one item or two.
המתכוין לחתוך את התלוש וחתך את המחובר אף על פי שנתכוין לחתיכה הואיל ולא עשה מחשבתו פטור מן החטאת שזה כמתעסק ולא אסרה תורה אלא מלאכת מחשבת כמו שביארנו כמה פעמים:
When, on Shabbos, a person had the intent to cut an object that was already severed from the ground, but instead, cut an object that was connected to the ground, even though he had the intent to cut, since his desired intent was not fulfilled, he is exempt from a sin-offering. He is considered as one who is merely busying himself and the Torah forbade only purposeful labor, as we explained several times.
But this is shver ["father in law" or "difficult"]. If he wanted to cut this cucumber from the ground and he did as he intended to the cucumber which was really attached ["נמצא"] then how can we say that it isn't מלאכת מחשבת. He did EXACTLY as he intended??!! That is why Tosfos say that it isn't מלאכת מחשבת only when he picked up a different item than he had intended. So what is the machlokes between the Rambam and Tosfos?
Rav Baruch Mordechai Ezrachi Shlita [Birkas Mordechai Shabbos p. 313] made the following chiluk: