It’s a popular way for sure. But different paths work differently - and Daoism is one of the more methodical ones.  
Yes - exactly. It’s a two way street. And ‘correct’ depends upon a lineage. There are different ‘corrects’.  
To achieve sagehood, a teacher isn’t required - it can be attained with a method alone (and a lot of luck, discipline and natural talent of course)... The DDJ is a method to become a shengren for example.
  But to go deeper, to the various stages of immortality for instance, a teacher is required because you need a living lineage connection.   
Yeah. Sadly true.   The rampant assimilation and homogenisation is sadly killing off any real growth of the spiritual arts.
  And we get shit like ‘Yoqi - combining the best of the ancient practices to improve your sex life in jus 8 minutes a day’. 
  So teachers fall under the spell of ‘we understand things better now’ and so we optimise, simplify, combine, reinterpret, hack, assimilate, scientifically validate, modernise, improve...
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