The doctor or lawyer, for instance, could use her or his income to invest in a medical start-up or buy shares of medical companies he understands such as Johnson & Johnson. Over time, the nature of compounding, dollar cost averaging, and reinvesting dividends can result in her or his portfolio generating substantial passive income. The downside is that it can take decades to achieve enough to truly improve your standard of living. However, it is still the surest path to wealth based on the historical performance of business ownership and stocks.
One of the best ways to build wealth is to get a handle on your finances by signing up with Personal Capital. They are a free online platform which aggregates all your financial accounts on their Dashboard so you can see where you can optimize. Before Personal Capital, I had to log into eight different systems to track 28 different accounts (brokerage, multiple banks, 401K, etc) to track my finances. Now, I can just log into Personal Capital to see how my stock accounts are doing, how my net worth is progressing, and where my spending is going.

In mid-2017, I sold my San Francisco rental property for 30X annual gross rent and reinvested $500,000 of the proceeds in real estate crowdfunding. I’m leveraging technology to invest in lower valuation properties with higher net rental yields in the heartland of America. With the new tax policy starting in 2018 capping state income and property tax deductions to $10,000 and limiting interest deduction on mortgages of only $750,000 from $1,000,000, expensive coastal city real estate markets should soften at the expense of non-coastal city real estate.

Whether you take a “distribution” (aka free-cash-flow) in the form of a dividend, interest payment, capital gain, maturing ladder of a CD, etc, you are still taking the same amount of cash out of your portfolio. Don’t fall for the trap of sub optimizing your overall portfolio’s performance because your chasing some unimportant trait called “income”.
Investing in bonds: Similarly, bonds are an attractive way to engage in passive income. Over a recent 45-year period, bonds funds, as measured by Vanguard Funds, returned 7.1%. Of course, there's no guarantee that investments in stocks or bonds will always work out well, investing in them is by far the surest way to generate money through passive income.

If you have specialized knowledge in a certain topic, you can put together an online course to teach others. For example, if you have experience in real estate investing, you can create an online course “Real Estate Investing 101”. The benefit of an online course is that once you create the course material, you can sell it to as many people as you want.