The Concept of Melachah
The concept of "Melachah," activity which is prohibited to a Jew on the Day of Shabbat, as noted in the Overview to Shabbat Section, means purposeful, creative interactions with nature. Let us explore some additional implications of the term "Melachah."
The idea of being purposeful, in this context, implies that that the act in question, in order to be considered a "Melachah," must have a positive purpose. Therefore, the act of demolishing a wall, for argument's sake, cannot be considered a "Melachah," even though one of the "melachot" (plural of "Melachah") is "Demolishing." It depends on what was the intention of the person who performed the action.
If the intention was purely destructive, then the act is not considered a
"Melachah," although it is not recommended as a way to spend one's Shabbat.
However, if the demolition was performed for the purpose of clearing an area for new
construction, then it would be considered as having a positive purpose, and the act is
considered a Melachah!
The aspect of "positive purpose" is derived from the context where the term "Melechet Machashevet" appears in the Bible, namely that of the construction of the Mishkan, the temporary structure which served the function of national religious and spiritual center of the Jewish People in Pre-Temple times. It turns out that the list of melachot corresponds exactly with the operations which were performed during the construction of the Mishkan (which are believed to be a model of all possible human creative activity). Since all the activities involved in the construction of the Mishkan were of a positive, constructive nature, we infer that likewise, the melachot of Shabbat, which are defined by "Melechet Machashevet," must also have that positive character.
An additional requirement for an act to be considered a full-fledged "Melachah" is the element of "kavana," or "intention;" also implied by the term "melechet machashevet," "Melachah performed with thought."
For an example of the above, assume the following: Tearing a branch off a tree intentionally is considered a Melachah. However, if someone had been napping close to a tree and, upon waking, stretched and accidentally tore off a branch, he did not technically perform a Melachah.
The final point in this very over-simplified discussion of Melachah is "psik raysha," meaning "cutting off the head." Background: To kill a living creature intentionally is considered a Melachah. But suppose, for example, that a parent, marooned on an island with only his son and a chicken, is being driven to distraction by his child on Shabbat. He decides that the only object that will serve as a toy for his child is the head of the chicken, and he proceeds to cut it off (pardon the unintentional grossness of the example) without any intention of killing the bird. Although it is possible to say that the element of "intention" is lacking here, the act nevertheless remains a Melachah, because experience has shown that it is quite impossible for the chicken to survive without its head (although one does often encounter individuals who seem to be running around in that condition).
To summarize, we've seen that the operative term for deciding whether a given act is a Melachah is "melechet machshevet." To be characterized as such, the act is considered a Melachah if it satisfies the following conditions: