From Chabad:
http://www.chabad.org/library/article_cdo/aid/604309/jewish/What-is-the-Jewish-view-on-gambling.htm"
In the Talmud,1 the rabbis take a dim view about gambling. Besides being a risky enterprise financially, and addictive, the rabbis say that the winner is really a loser. Morally speaking that is. How so? Because the fellow with the inferior hand wasn't expecting to lose. Therefore, the loser relinquishes his money reluctantly—it's being taken from him willy-nilly, and he is getting nothing tangible in return. In simple English, it's a bit like stealing.
That's not all, though. Gambling, whether betting on horses, roulette or cards, only gives the illusion of contributing to the local economy. In the end, though, it contributes nothing of value that endures.2
Rabbi Eliezer Danzinger for Chabad.org
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From Ohr Samayach Ask the Rabbi:
http://ohr.edu/ask_db/ask_main.php/141/Q1/"
Lee Vyner wrote:
Dear Rabbi,
The Talmud says that someone who earns their salary from gambling is invalid as a witness in a Jewish court because the person who lost doesn't really pay with a full heart. Therefore, the money that he wins is considered theft. If so, what about the stock market? People literally gamble as to the price of a particular commodity or stock, and the person who has a holding of this particular item certainly does not want the price to deteriorate. So is someone who gambles on the stock market invalid as a witness? Lots of love, Lee Mandy and Laivy Avraham (the cutest baby in the world bi'h) Dear Lee and family,
Stocks are a legitimate investment. Buying stocks is essentially no different than buying diamonds, land, or wheat. True, the stock market has an element of risk, but don't confuse `lost wages' with `Las Vegas.' Do you know of any business which involves no element of risk? If so, do you know their phone number?
Gambling, on the other hand, is a game where each person hopes to guess the right number on the dice, or pick the right horse. Gamblers are invalid as witnesses in a Jewish court.
The Sages of the Talmud differ as to why gamblers are invalid. According to one opinion, someone who wins a bet is like a thief, because he collects prize money that he didn't `earn' and to which he has no true legal claim.
According to another opinion, only a professional gamester -- who has no other source of income -- is invalid as a witness. His integrity is suspect, because he spends his day in pursuits which contribute nothing to society. According to this opinion, someone who gambles only part-time can be a valid witness, provided he's involved in some productive activity.
The following story is told about the Chafetz Chaim: A man once asked the Chafetz Chaim to bless him that he should win the lottery, but the Chafetz Chaim refused. "But you give blessings to people who gamble on stocks, why not when they gamble on lotteries?" the man asked. The Chafetz Chaim answered that he gives blessings to stock investors because if the stock goes up, no one loses money. But blessing one lottery ticket is a `curse' upon the other lottery tickets.
Sources:
* Talmud Sanhedrin 24b
* Shulchan Aruch Choshen Mishpat 34:16
* Story thanks to Will Shulman
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