הלכה על אבות 2:9

And do not wonder to say, "And how is it in the hand of a man to prevent his heart from desiring the storehouse of every delightful vessel that he sees in the hand of his fellow, whereas he is totally empty of them; and how does the Torah bring a prevention about that which is impossible for a man to uphold?" As the thing is not like this, and only silly evildoers and those that sin with their souls say it. As truly, it is in the hand of a man to prevent himself and his thoughts and his desires from anything that he wants. And [it is] within his control and his cognizance to distance and to bring close his want for all things according to his will. And his heart is given into his hand; to anything that he wants can he incline it. And God - in front of Whom are all hidden things - 'searches all the rooms of the belly, sees the kidney and the heart.' There is not one from all of the thoughts of man - little or big, good or bad - that is hidden from Him; and it is not covered from His eyes. [Hence] He will bring vengeance upon those that transgress His will in their hearts, and 'safeguards kindness for the thousands [of generations] for those that love Him,' who dedicate their thoughts to His service. As there is nothing as good for a man as good and pure thought, as it is the beginning of all actions, and their end. And this is apparently the matter of the 'good heart,' that the Sages praised in Tractate Avot 2:9. And the proof that these two negative commandments are different in their content and considered two [distinct commandments] is that which they said in Mekhilta DeRabbi Shimon bar Yochai 20:14, "'You shall not covet your neighbor's house' (Exodus 20:14), and later it states, 'and you shall not desire,' to make liable for the desire on its own and for the coveting on its own." And there, it [also] says, "From where [do I know] that if he desires, his end will be to covet? [Hence,] we learn to say 'you shall not desire' 'and you shall not covet.' From where [do I know] that if a man coveted, his end will be to steal? [Hence,] we learn to say 'They covet fields, and steal' (Michah 2:2)."