The so-called passive income (Passive Income), is not need to spend much time and effort, and do not need to take care of, can automatically get the income. At first glance, it looks a bit like "getting something for nothing", but in fact, it often takes a long time of labor and accumulation before getting "passive income". Passive income is a necessary prerequisite for financial freedom and early retirement.

Financial freedom is the state in which a person does not have to work hard for money to make ends meet. Simply put, the passive income generated from one's assets must at least equal or exceed one's daily expenses, and if one enters this state, one can be called financially free.

Financial freedom applies only to a small population; if the vast majority of people tried to reach financial freedom, the system would inevitably collapse. The theory suggests that the passive income on which financial freedom depends, comes from the active income of others.