Learning happens whenever has a moment of insight. It sometimes comes from trial and error, but often results from listening or reading. Those are among the miraculous human processes by which knowledge, once gained by one person, can be transmitted to others without having to be rediscovered from scratch. Unfortunately, falsehoods can also be transmitted this way—on a massive scale.
The more abstract the topic, the greater this danger. If parents teach children that hemlock is not poisonous, reality will correct the error tragically. If, however, certain economists teach that effects precede causes (consumption precedes production) and most people wishfully believe them, there is no immediate and broadly visible corrective feedback. Though we suffer the results of such false ideas in the form of economic turmoil, war, and impoverishment years after the causes were set in motion, without a correct understanding of economic theory and history, we cannot grasp the true sources of such problems.
We do not have to rediscover everything from scratch. We do have to recheck what we learn and come to our own decisions. Advice from people we trust can be helpful, but can never be completely reliable. Even if a trusted authority seems right about many things, they are often mistaken on some points. Worse, some “trusted authorities” are thoroughly trained and immersed in misleading webs of falsehoods.