While this is a key to philosophy, it is also a key to one's personal psychology. Without correcting this view of oneself, one cannot be happy. (See the definition of what comprises happiness: Happiness) People waste so much energy on having a dysfunctional view and set of beliefs about themselves. If one focuses on getting realistic views and beliefs about oneself (and mankind), then one will save a huge amount of resistance and effort fighting (false) barriers. Also, one will not put artificial limits on oneself by a false view of oneself.
This is vital, critical, essential to living a good life. It should command your time until it is fully and effectively designed and owned.
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Some of the highly erroneous beliefs that people have are discussed in the following pieces:
Personal
Who I Am - Without this understanding, you'll spend time on useless defending or self criticism. Knowing that "I" is not what you think is vital to self understanding.
I Am Powerless - A belief that people develop, but with no truth to it. See why.
I Am Not Good Enough...Not! - This is a very correctable stance - and it must be corrected if one is to be happy. It's not optional. It is always fixable, though.
Humans overall
Practices and viewpoints that affect one's view of oneself