There's quite a few articles about the difference between thinking "positively" and being constructive (which is ultimately what the secret fails to differentiate to people).
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Also, this is an interesting blog on who was the motivation behind the author.
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What they call "positive" thinking is actually just superstitious thinking and what they call metaphysics isn't metaphysics proper but rather a form of "pop" metaphysics. Psychologically, it may make sense to be more positive so you "attract" better people into your life. BUT... this idea, taken to the extremes it is in the movie, is misleading and just impractical. For example, you don't create abundance by wanting things. People usually want things to fill a void and you can't be content and in an abundant mind set while constantly wishing for things. Even psychologically, it just doesn't work that way.
It is true that once you put on, for example, the "rich mans" cap you start seeing financial opportunities you've never noticed before, but that's not a secret. Its common sense which can be explained practically without all the rhetoric of pseudo psychology.
I mean, the people who enable the magical thinking that surrounds this movie are either just being wishful or simply misled by semantics. You don't get a check in the mail from wishing for it. There are reasons checks come in the mail and it can usually be explained on a practical level instead of just saying "I've manifested cash!". lol. Uh, no you didn't. You co-operated with the world in such a way that enabled you to receive that cash. It doesn't just get created from the ego by wishing it too and then sitting on your butt.