We are continuing in siman 9. The Chayei Adam is discussing one who begins a large project, such as writing a letter, harvesting a field, or weaving a basket, but only performs a small part of the project. One could mistakenly think that since they have not completed the entire project, it is considered a chatzi melacha and one would not be chayav mideoraysa. The Chayei Adam explains that this thought is incorrect, because once a person performs an amount of melacha which amounts to the halachic shiur for that melacha, they are chayav. The Chayei Adam gives the example of one who wishes to write a letter. Once they have written two letters, they are chayav.
The Gemara considers this point a chiddush. The Gemara explains that one might have thought that the concept of meleches machsheves would mean that if a person does not accomplish their intended melacha, they are not chayav mideoraysa, because the action performed must be isavida machshavto, what the person intended to accomplish. However, the Gemara learns from a pasuk that one is chayav once they write two letters. One cannot reach their intended result (writing an entire document) without this small step, so even though they did not complete their intent, this is an entire unit towards completing their intent. Similarly, if one got called away after writing two letters, they are chayav, even though they have ceased from fulfilling their intent, because they have completed the minimal unit of melacha for this issur.
Rashi makes an interesting suggestion and writes that maybe one is only chayav if they pause in a place which is normal to pause. If they pause in a place where it is not normal to stop, maybe one would not be chayav mideoraysa. For example, if one is weaving a wicker basket, the basket is held together by the tension of building up the walls of the basket. If one stops after weaving one row, it will not hold well. Therefore, he understands that this is why the Gemara says one is only chayav for weaving when they weave two rows. Even if a person would not weave an entire basket in one sitting, they would minimally weave two rows, so if one stops before that, they are not chayav mideoraysa.
The Gemara gives another example of a person throwing an item in a reshus harabbim. The minimum shiur which needs to be traversed in order to be chayav is four amos. One can traverse the shiur by walking or throwing an item. If one had intent to throw the item 8 amos, and only ends up throwing it 4, since having it 4 amos away is not the intended result, the melacha is lacking in isavida machshavto and is not assur mideoraysa. Although it appears to be part of the process of getting to 8 amos, the person did not have intent for a partial action, so it is patur mideoraysa. This idea would not apply to something like harvesting a field, where every unit of the field which is harvested already brings a person closer to their intended goal.
Summary
The concept of isavida machshavto applies to situations in which performing anything less than the entire melacha is worthless to the person. In such cases, the melacha is lacking in meleches machsheves and is patur mideoraysa.
When each partial step of the melacha is worthwhile to the person, the concept of isavida machshavto does not apply, because each step is considered a unit of meleches machsheves, and one will be chayav once they perform that partial step, as long as they have done the shiur given by Chazal as the minimal shiur.
