Ultimately, society as a whole and business in particular are all about relationships. Then, success - or in the more personal area, happiness - is measured by how well one can establish and maintain personal and long-lasting relationships.
Within the traditional business context, the primary relationship is usually seen to be that of a company with its customers. But let's turn this around:
The primary business relationship is that of a person with a company of his choice.
If, and only if, this person chooses to engage with a company, he or she becomes a customer.
Customers are the shadows in Plato’s Allegory of the Cave: they may be interesting to watch, but will never enable you to fully grasp the real person they represent, let alone understand his behavior.
The crucial aspect of "outside in" thinking, is therefore that companies should try and picture the person rather than the customer. Who was that person before he became a customer? How did he become a customer? What becomes of that person when he no longer wants to use our products or services?...
A better understanding of the individual will lead to a better understanding of the customer, potentially true intimacy, and who knows, loyalty and advocacy may be peeking around the corner.
Strive not to be a success, but rather to be of value.
Albert Einstein
Today, business success should be described as how well a company can identify and understand a 'customer-typical' person, get in touch with him or her, turn him or her into a loyal customer, exteract fruitfully, and understand why that person moves on some day.