One of the עשרת הדברות (known in English -- because of a decidedly purposeful attempt by the non-Jewish translators or our Torah to corrupt its meaning -- as the Ten Commandments) is, לא תגנוב/Thou Shalt Not Steal.
It says nowhere in our Torah that one should eat a healthy diet, nor does it say anything about wearing clothes, nor breathing, nor... nor... nor... Of course not, you say, those things are obvious and don't need to be said to normal, rational human being; certainly not commanded! Right; and a normal, rational human being similarly is not in need of being reminded -- certainly not commanded -- not to steal. So what's going on? A lot.
First, the prohibition against stealing includes deriving a benefit from someone who steals (cf תרגום יונתן). Of course (again and again, we will need to add "of course") one is not allowed to do business with stolen goods, but even having unnecessary favorable social interactions with someone who steals is prohibited. But there is more.
What does the Torah call stealing (besides the obvious, of course)? Using something without the owner's explicit permission is stealing. Even more: using something for a purpose other than that for which you asked permission is also stealing. Suppose, for example, you borrowed someone's pen without asking. Even if you know he won't mind and even though you have in mind to reimburse him for the ink you used. That's great, but you still would have stolen. Of course he'll forgive you. That's great, but you still would have stolen. Suppose you asked him to borrow his pen to make a few notes in your gemara, then -- before returning it -- you wrote a check to a m'shulach. You have done the wonderful mitzvah of giving charity; but you tainted that beautiful mitzvah with the stain of theft.
One more: you loan your friend $100 for a month. At the end of the month, no payment is in sight. After two or three months, you figure it is a lost cause. After six months, you say to your chavrusa with a sigh, "I am never going to see that money from Ploni; I'll just forgive the loan so at least he isn't transgressing the sin of not repaying a loan and I'll at least get the mitzvah of charity." The next week, who should show up, but Ploni -- with $100! You are so touched and you feel your faith in human nature is restored. One thing: Don't accept that money as payment, because you forgave the loan; that's not your money. Accepting that money as a payment is tantamount to stealing. (Of course, you can tell Ploni that you forgave the loan and that you would happily receive a gift from him, but you cannot let him believe that he still owes you the money.)
That's just scratching the surface of the implications of one of the עשרת הדברות.
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