I find this principle very useful in my thinking, particularily when you want to determine if something is legit. It is based party on elementary logic laws and partly on a rule of thumb. I call it the principle of triviality:

Any self-consistent body of knowledge can be reduced to a number of clear elementary postulates from which everything else logically follows. Therefore, if the foundational postulates of a system are not immediately apparent, the system is likely not self-consistent.

Also, the number of those postulates is usually small (this is the rule of thumb part).

Simply put: if you cannot explain something using very elementary language and constructs, then there is probably inconsistency in it somewhere.

Example: most religious doctrines, political ideologies, astrology, cryptocurrencies etc.