As an struggling young Engineer (back in the Carter era) I bought anything I could renovate then rent to justify paying the 18% interest. I never took vacations but worked on my properties all in the pursuit of passive income. I drove junk for many years & many months I just got by on credit cards. My friends & colleagues were amused by my ‘stupidity’ but most are still working to make enough for retirement.
"But for our jobs, for how we make money, I think many of us — most of us — have one way we make money. It can be very risky. I've really come to believe, and have done a lot of research over the years, that one of the best ways that we can create real, legitimate professional stability for ourselves is by choosing to cultivate multiple income streams.
Why did P2P lending get a liquidity ranking of 6? It is quite possibly the most illiquid investment option you listed. You said you rank liquidity by “difficulty level of withdrawing your money without a massive penalty”, and for Lending Club notes, it’s not only difficult and extremely time consuming to sell all of your notes in their super illiquid market, but you would have to sell your notes at large losses to hope to get others interested in buying your notes. On top of that, it is impossible to withdraw your money any other way other than just waiting for interest/principal to pay off every month until maturity in 3 to 5 years. You can’t just one day tell Lending Club “I want to quit, please give me my money back.” One can even argue that it is less difficult to sell a home (in order to “withdraw” the money invested) than to withdraw all of their money from a P2P loan portfolio because it is very possible to sell a home before 3 to 5 years.

For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.