The 266th prohibition is that we are forbidden to occupy our thoughts with our desire for someone else’s property and to develop a craving for it, and dwell upon it, since this will lead us to carry out a plan to acquire it.
These two prohibitions [lo sach’mod and lo sisaveh] do not have the same goal.2 The first prohibition, lo sach’mod, forbids buying someone else’s belongings, whereas the second, lo sisaveh, prohibits even having the feeling of desire and envy.
The Mechilta says, “Here it says ‘Do not envy [lo sach’mod] your neighbor’s house,’ and later it says, ‘Do not desire [lo sisaveh] your neighbor’s house.’ This makes the desire and the envy separate prohibitions.” It also says there, “How do we know that a person’s desire will lead him to envy? Since the Torah says ‘Do not desire’ [lo sisaveh] and ‘Do not be envious’ [lo sach’mod]. How do we know that if he is envious he will ultimately commit robbery? Since the Torah says,3 “They envied fields and robbed them.”
The explanation of this passage is as follows: If one sees a fine object that belongs to his brother, and allows his thoughts to gain control over him, and develops a desire for it, he transgresses G‑d’s statement (exalted be He), “Do not desire” [lo sisaveh].
Then his love for the object will become stronger and he will carry out a plan to acquire it — coaxing him and pushing him to sell it or to trade it for something better and more expensive. Should he reach his goal, when he acquires it, he also transgresses the prohibition, “Do not be envious,” since by pushing and scheming he acquired his friend’s object even though he had no intention of selling it. At this point, he has transgressed two prohibitions, as we have explained.
If, however, the owner, because of his love for the object, refuses to sell or trade it, then his great desire for it will cause him to take it by force and violence. At that point he also transgresses the prohibition, “Do not commit robbery.” Think about this in relation to the story of Achav and Navos.4
We have therefore explained the difference between lo sisaveh and lo sach’mod.