He means that we fool ourselves any time if we dont LEARN if it is the time to trust only ourselves or the time trust an other...
We are the easiest person to fool because we dont know why it is time to go alone or to listen others... Simple...
No, you wrote in utterly vague circles, never landing on his actual point.
His point clearly had to do with what separates the scientific endeavour from everyday level inferences.
When Fynman says "The first principle is that you must not fool yourself, and you are the easiest person to fool."
That is clearly a warning about the influence of human BIAS in distorting and guidling our conclusions. "You are the easist person to fool" is a reference to how easy it is for us to filter explanations and evidence to fit our desires or biases. In fact, we are easy to fool through various pitfalls in thought, even when we are trying to not be biased. When YOU are the one doing the testing YOUR actions and interpretations will have a blind spot of your own bias.
He admonishes us therefore The first principle is that you must not fool yourself," which means we have to incorporate guardrails against fooling ourslef in to our methods of inquiry.
This is so obvious it's just hard to believe folks like you and rodman can't just state what he meant.
Since our biases form such an obvious, first problem in interpreting results, this is why there are various methods of mitigating the influence of bias in scientific testing. It's why for instance many therapeutic trials are done blind, double and even triple blinded.
It's why you want to have a hypothesis that is testable by other parties, looking to prove your hypothesis wrong, themselves using safeguards against their own bias effects.
This has OBVIOUS implications for testing audio claims. If for instance sighted bias is a known confounding variable - a prime way of FOOLING YOURSELF - then Feynman's admonishment clearly indicates you should find a way to rule out that way of FOOLING YOURSELF. Job ONE of the approach he is advocating!
This is why most of the scientific level of research on human perception in general, and much that is available on the perception of audio gear (e.g. the research often cited by Floyd Toole) is done with controls for those variables so the FOOLING YOURSELF part is mitigated as much as possible.
Of course neither you nor anyone else no this forum needs to do scientific-level rigorous research in order to enjoy the hobby or buy whatever you want. But if someone is invoking Feynman in a thread that clearly entails the relevance of science to audio, then at least get what he was saying. You can ignore it...but at least understand it.