Okay, let’s go with that. And let’s say that someone wants to judge whether or not some - hypothetical individual - is a success.
The person is accomplished enough - well educated, makes a decent income, does a job that the person thinks is worthwhile and that makes a positive difference for others, married to the same partner multiple decades working through problems, by most standards would be called a good parent, well respected by peers, physically fit, says true to his/her ethical standards, so on … but how do we know what that person’s personal potential is or has been?
Maybe if that person could have really devoted him/herself to the violin and become a world class violinist but never tried that, even quit music training early in childhood. In any case whatever music potential this person has is completely undeveloped and unfulfilled!
Maybe if the person had worked harder or chosen a different career path (s)he could have made more money. Maybe if a chance meeting had occurred they’d have taken a path making them more renown in their field.
Maybe given what chances they had by accidents of birth, the era and culture they were born in, the opportunities available, the family support they had, the peers they were exposed to, so on, they had potential to accomplish more academically, more financially, more fame … how does the person know? How does an outside observer know?
How can one ever judge whether or not someone “is fulfilling personal potential” when we are always guessing as to what that potential is/was? Anyway fulfilling one element of potential to some degree might mean not fulfilling another.
Nah. That “natural definition” is a reasonable mindset to use to set goals for ourselves - “I want to be the best me I can be.” - but knowing what the “best me” means, is and would potentially be? Those are the interesting things for each of us to decide before we embark on discussing how to achieve it.
I would posit that many (I don’t know about “most”) people are not successful because their perceived “best me” is off-target and they are chasing things that will never get them to where it actually is, and/or because they have no, or at least insufficient, actual interest in the goal.
Can you honestly look back and say that you have done the best you reasonably could with what you’ve been given in life, your innate strengths, your innate weaknesses, your opportunities chance or otherwise, to do the best you could at achieving that which you feel is important? That can be a being a good spouse, a good parent, a good friend, a good colleague, a good citizen of the country or the world, career or other interest accomplishments, and/or whatever is what you personally value. Then you have good ground to call yourself a success. IMHO.
If being a good friend is important to you and you fail to be one in pursuit of “high quality” acquaintances … then you are a failure. IMHO.