Can There be a 'Rational' Narcissist?
Narcissists are generally perceived as delusional (and hence irrational) individuals; however, would an individual who is universally deemed the greatest in their domain, be delusional for having an ideology consistent with external perceptions. For example, if Isaac Newton crowed about his intelligence, would he be delusional?
If Beethoven boasted about his creativity and referred to himself one of Earth's greatest musical talents, would anyone dare refute his belief and think him as irrational?
In what way could their ego be considered inflated? In what way could Isaac Newton's self-perceived importance, by virtue of his intelligent, be considered non-respectively grandiose?
(Note: Both examples assume that the individuals in question meet the criteria for narcissistic personality disorder and are not necessarily representative of their actual character. Additionally, the intention of this debate is to analyze the attributes of delusion and grandiosity that psychologists maintain accompanies a narcissist.)
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Like you said, the clinical definition of a narcissist is somebody with delusions as to their Intelligence or power. Or wit or charm or whatever.
So no, a deluded person cannot be rational. At least not concerning the same issue they're deluded about.
Your examples of Beethoven and other talents are off. Since a person can be a rational but arrogant of confident person. He just knows his strengths! And if he is the best in that field then this is not delusion.
The DSM-V definition of NPD excludes the patient being rational.
The answer to your question is No.