1. Doubt what you see. Because your observations are filtered through your perception, you risk being fooled if you accept your beliefs too quickly.
2. Test to be sure your beliefs are based on natural principles. Test them at least two or three times to make sure they lead to similar outcomes and, therefore, represent reality, not delusions. Example: Do people really buy your product because they think it does a better job than the competition? That’s what you think, but you can’t really know until you talk to lots of customers.
Da Vinci also cautions us to reject beliefs that don’t flow from direct observations. Example: He rejected alchemy because it had no basis in nature. Had any base metal turned to gold in nature? No! So why pursue the idea?
Apply that yardstick to business fads, and accept only those ideas based on hard-won observations of how people really think and behave.
— Adapted from The Notebooks of Leonardo da Vinci, Oxford University Press