A few months ago, a bright young man named Charles came to see me for advice about a business he was starting. The business, he said, was a hedge fund of cryptocurrencies like bitcoin. There were actually more than 1,500 types of cryptocurrency at the time, and the number had been growing fast, but he said he was trading only the few that were well-constructed and likely to last. "What do you think about cryptocurrency?" he asked.

I've heard some version of the question "What do you think of my idea for a business?" over and over for the past 50 years. First-time entrepreneurs always want to know what I think. I gave Charles the same answer I've given all the others: "It doesn't matter what I think. All that matters is what you think."

"But you must have an opinion," Charles insisted.

"Sure, I have an opinion," I said. "So what? I may be wrong." And then I told him a story.

It happened in early 1969, when I was a newly minted, 26-year-old lawyer with my own practice in Brooklyn. A guy named Richard Nader came to see me. He had an idea for a rock 'n' roll concert business featuring performers from the 1950s. He was planning to stage the first concert at the Felt Forum in Madison Square Garden. He wanted me to put up $25,000 to help fund it. "What do you think?" he asked.

I told him I thought it was the stupidest idea I had ever heard. Understand, the country was still experiencing the British Invasion set off by the Beatles, who scored three U.S. No. 1 hits in 1969--five years after they first topped the Billboard Hot 100 with "I Want to Hold Your Hand." Motown was also going strong. I couldn't imagine that enough people would want to hear a bunch of washed-up rock 'n' roll bands to fill the Felt Forum, and I wasn't alone. Nader had already spent four years trying to interest well-known music promoters, including Dick Clark, in his idea. After striking out with everybody else, he finally managed to borrow the money he needed from a furniture manufacturer and put on his first two Rock & Roll Revival concerts on October 18, 1969, with performances by, among others, Bill Haley and His Comets, the Coasters, the Shirelles, and the Platters.

Both shows sold out. So did almost all of the 25 oldies concerts he went on to produce at Madison Square Garden. They were so popular, in fact, that they had to be moved from the Felt Forum, which could accommodate about 4,500 people, to the main arena, which held up to 20,000.

Over the next 40 years, Nader took his oldies shows to giant venues throughout the United States and Great Britain. He even produced an oldies movie based on the concerts. A company bearing his name that he started in 1989 is still staging oldies concerts today, nine years after his death.

In the early days of his business, Nader would always send me two front-row tickets to his concerts in Madison Square Garden. It was his way of reminding me how wrong I had been about "the stupidest idea I had ever heard." That idea wound up making him millions and millions of dollars and transforming the rock 'n' roll concert business.

I got the message. Since then, I have never told entrepreneurs starting businesses what I think of their ideas. On the contrary, I have urged them as forcefully as I can not to ask for or listen to other people's opinions of their ideas. The world is full of naysayers happy to tell you how crazy you are to take a chance on a business.

I think my story convinced Charles that there was no point asking me about his idea. I told him I was very willing to offer my thoughts on how he might implement the idea and what was the likeliest way to raise the money he needed. He thanked me. I asked him to keep me posted on his progress. I'll let you know what happens.