I support you point in general, but I am curious, that you say "painting is worth X because two people agree". What if they disagree? I'd be interested to read some thoughts on that. Even better - proper theories.
I think it goes even further than what the previous comment said. The value of the painting is different for everyone and even for the same person it will change in different circumstances (value of a cup of water at home or in the desert).
When a transaction happens the two people very probably don't agree on the value. Rather, the seller values the painting at "less or equal" of the strike price and the buyer values it at "equal or more". The transaction happens if those two regions overlap somewhat.
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At the end of the day, something has value because two people agree it does. One painting is worth $100million and another $100 because two people agree that’s what it’s worth. Why two people agree something has value is the matter of endless debate. But I assure everyone reading that the commenter I am responding to is not certain to the answer, and actually no one really knows. This is philosophy, we can all have opinions.