@Daddy
1. Depends on the person.
2. My point is: You define your meaning, and if can't define meaning, you don't define meaning. If you know it, you know, if you don't, you don't.
3. Such hypotheticals hold little interest to me, as a single example cannot encompass the neurological make-up of billions of primates.
There is no almanac that I can slam open to reference person no. 84342943213 and decide, "Yeah, that's a meaningful life right there."
But person no. 43027025? "Nah."
I am not one of those above persons, their lives are not mine, their fleshbags are their own to pilot. How the hell can I tell whether they live meaningful lives? Only my own meaning can be determined, because only I can determine it.
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From the outset you ask: How do you justify existence? I answer: What is there to justify? If you die in a contented state, you die in a contented state. If you die a sad and broken human with regrets and stress tearing your mind apart, you die a sad and broken being. You are what you are, and if you don't like what you are, you might work hard to change yourself. If you fail to change yourself, you fail.
Do or do not. Why justify? Seems like a useless loop in the equation. Sure, if you believe in a greater cosmological entity that expects certain mores of action from you, or if you feel the need to prove yourself to the universe in some way even if you are only an atom in it, then you might have some drive for justification.
For me though, it just seems a waste of energy. Write a book, adopt a puppy, blow up a bridge, create meaning. What is there to justify?