People of the same trade seldom meet together, even for merriment and diversion, but the conversation ends in a conspiracy against the public, or in some contrivance to raise prices.

— Adam Smith, An Inquiry into the Nature and Causes of the Wealth of Nations

I first heard this on Planet Money #829. And I found the whole paragraph as following:

People of the same trade seldom meet together, even for merriment and diversion, but the conversation ends in a conspiracy against the public, or in some contrivance to raise prices. It is impossible indeed to prevent such meetings, by any law which either could be executed, or would be consistent with liberty and justice. But though the law cannot hinder people of the same trade from sometimes assembling together, it ought to do nothing to facilitate such assemblies; much less to render them necessary.

— Adam Smith, An Inquiry into the Nature and Causes of the Wealth of Nations

When dentists in N.C. tried to shut the teeth whiteners out of the industry, the main problem was not only the dentists themselves but also the state government. Well, seems that Mr. Smith foreseen them all.

Hats off!