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This is an interesting and important topic, Bruno, and for once I'd say that I can fully understand what Richard is saying here.
Being a person who has for a long time been prone to self-deprecation, I entirely agree

It's good to finally be getting rid of my insecurities, and doing away with the need for self-esteem (which is just an unreliable antidote to insecurity).
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I'm not necessarily saying those exercises are helpful, but I do know that they shrink your sense of self. Everyone has been humiliated or embarrassed at some point in his life. One might say of such an experience: "I felt like I didn't exist" or "I felt about 2 inches tall". The self has been shrunk, albeit against the will in most cases.
Ah, but it hasn't been shrunk at all! This is an important point to grasp about what Richard is talking about (both in the text quoted above, and in other similar pieces): feeling like I don't exist, i.e., feeling humiliated, abashed, ashamed, humbled, etc, is completely self-preserving, self-sustaining, self-serving, etc. Although it might not appear that way.
To wit: feeling very small, feeling that one doesn't exist, is feeling "me" as "non-existent/small/humble," rather than
not feeling me at all. They can be distinguished because the former is very uncomfortable, and the latter is not (in fact, coupled with sensory perception, being out of the way is very delightful).
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How one responds to this is more important. The ego can look at the self-imposed, (or other-imposed) humiliation and be proud of the fact. "Look how humble I am!", "Look what I can endure in the name of Buddhism!". That is just the ego trying to recover from the insult, as Richard points out.
Now do you see that Richard actually means that self-humiliation, regardless of one explicitly feeling proud of the fact, is ego-centric? It is ego-centric in itself, irrespective of whether one secretly brags about it or not.
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But what Richard misses is that the same insult can be handled in an entirely different way that allows the ego to be diminished. You can notice where that humiliation is felt in the body, and you can notice how the internal dialogue starts up and goes into overdrive, replaying the incident, "what a bastard so and so is", what you should have said, etc. To be able to witness those moments without 1). reacting, or 2). indulging in ego defense mechanisms, is what's required. He only talks about ego defense mechanisms, which is a fraction of the possibilities available to an individual.
It is a very good practice to become aware of how "feeling insulted" can lead to internal dialogue and other imaginative pursuits. It is an even better practice, as you suggest, not to feel insulted in the first place. If you feel insulted, the ego has already happened, and you are left to deal with the consequences.
Enormous power can be accessed by shrinking the self. I know that much from personal experience. And I'm not talking about harnessing the anger that emerges, no. That's just the ego bouncing back, because anger is its most powerful tool for self-preservation. Something emerges that is very powerful, and has no anger in it.
Yep, I agree that self-humiliation can be very powerful. It is no surprise that mystics from all over the world in all times in history made use of self-flagelation in their mystical practices.
But the point here (which I am repeating over and over to be clear) is that: self-humiliation does not actually shrink the self, quite the opposite. The proof that the self is still there, that it is enourmously there, is that "enourmous power can be accessed by [self-humiliation]" (for who would want "enourmous power"?).
If one is worthy of despise, one is somehow not "just another human being." Self-humiliation is a delusion of anti-grandeur.