Capital is the money put into the business by owners/shareholders. It must be conceptualized as a liability because this is an obligation to the owners/shareholders. In the event of the business closing, this capital is considered a "debt" to the investors, just as an unpaid bill is a debt. Hence, both are liabilities. We find it easy to think of accounts payable as a liability because we have some good or service that we know we owe money for, but we tend to forget that we "owe" the capital, too. There is always a fiduciary responsibility to shareholders, so this is not money we can do what we please with.