It's one thing to know your business. It's another to know your customer. " what you say ?

Agreed with the statement and the person who understands well both customers and business will succeed at the end. 

You have  not known your business if you have not known your customer.Business Means Customer and Customer Means Business.

Both are correct but there is a third point in order to achieve : Know yourself . 

Business perfections are based on corrections as to the demands/needs of the target group. First you create a business on the basis of expectations, those expectation start fulfilled by a small audience first and later on the group turns to be bigger in size.....now you know your customers and their requirements;  and adjust your business by corrective expansions that fulfill your customer needs to retain them and add value for their expectations.

It is not enough to know everything about your business, about your products or services, about your SWOT, about your strategies and plans. If you don't know your customers well, you won't be able to do a successful business.

Doing business is nothing more than having a product/ service that should be connected to customer by lines of values and benefits. If you do not know your product/service and you do not know your customers, how can you define the values and benefits?

Knowing your customer is very essential if you want to know your business. Furthermore, you should know your customer's likings and expectations before you can say you know your business well.

Successful entrepreneurs know with exactness the wants, wishes and buying 
behaviors of specific individuals. They have analyzed the size of the market or the number of potential customers that fit the target profile. But further still, they know their customers’ names, ages, genders, incomes, home and Internet addresses, professions, education, associations, and marital status, number of children, hobbies, their tastes and interests. They grasp what their customers watch, read and hear. They understand their likes and dislikes. Knowing a customer to this depth is one of the key characteristics of highly successful business leaders

Both are very much connected. Your business is the demography of what they need and like.

yes true, but know yourself and your product/service also important