A request I received this morning reminded me of a little-understood but immensely important principle of advertising. I call it the “infinite-universe” principle. If you know this principle and apply it to your business, you can make a lot more money and avoid a lot of needless bickering.

The particular incident I’m speaking about involves two clients. One — who we shall call George — owns a magazine. The other — who we shall call Sally — has a contract with George to promote her product line to the magazine’s subscribers. The arrangement has been working well for both of them. But when George got the idea of selling other kinds of products to his subscribers, Sally objected. She feels that the new line of products will take away from her sales. George wrote me, asking for help.

“Sally believes in a zero-dollar theory of advertising,” George wrote — “that if we sell this new line of products, it will depress sales of hers.”

George has a good point. A longtime listener to my boisterously provided ideas, he understands that the amount of dollars his readers are spending on Sally’s products is not the total amount they would spend if given the chance. Exposing them to other products should not, in and of itself, depress sales of her line.

But Sally’s perspective is understandable too. If you’re doing well with an exclusive access to a given market, why would you want to let others in? How could it do anything but dilute your sales?

Surprisingly, the truth is that sales usually increase when you increase advertising to a limited marketplace. Instead of creating more but smaller pieces of the same-size pie, additional selling usually creates a larger pie.

That’s what happens when your restaurant monopoly on Main Street is suddenly challenged by two more restaurants on the same block. For a week or so, your sales may dip a little — but in the long run, you’ll do much better.

The same holds true with almost any retail operation. That’s why it’s so common to see like-kind operations in the same area. You often see this with antique stores, furniture stores, car dealerships, and art galleries — to name a few examples.

The size of a market is not limited in dollar terms. The same number of people that spent $1 million one year can spend $1.2 million the next — if they are stimulated to do so. Likewise, a market of $1.2 million can shrink to $1 million or less when promotional activity is slowed.

There is actually a universal principle underlying this. Everything in life is expandable — happiness, misery, pleasure, pain, even time if you believe Einstein. If you think of anything as being limited and conduct your life accordingly, it will be limited. But if you understand that you can get more by opening yourself up to more, you’ll get more.

But let’s get back to money.

You can make more money for yourself and your business if you recognize that the amount of buying your customers will do is not limited. If you provide them with more selling stimulation, they will probably buy more. The limitation usually comes from the seller’s side. Most marketers have a limited number of products to sell and a limited number of ways to sell them. When all those products are sold in all those ways, they think, “Well, that’s it. This market is tapped out.”

But it’s not. What’s happened is that one particular marketer has reached his limit. By inviting someone else in to market a different set of products in a different way, additional sales can be generated. In the case of George and Sally, Sally will be better off if she realizes that it probably won’t hurt her one bit — and it may even help her — to allow additional products and promotions to reach George’s magazine readers. So long as the selling messages aren’t the same, there should be no loss of sales — just a better deal for George and more opportunity for his subscribers.

Don’t make this mistake in your own business life. Most business universes are expandable. If a new guy comes into your neighborhood and starts competing with you don’t assume that you are in danger. Every dollar he spends advertising his wares is a dollar spent advertising your wares too. The greater the combined promotional enthusiasm there is the stronger the interest will be, particularly in the minds of people who were not buyers previously.

This is the kind of contra-intuitive marketing secret that creates fast fortunes for those who know.