Beautiful this explanation Alex, thanks!
So I would continue, for having very clear in my mind how to "build" the right purpose.
I'm PRing financial books, trading books, I explain now my steps for finding a valid purpose:

My 1st objective is to develop a trading system which allow me to gain 20% at least. So the purpose can be "I'm PRing these books because I want develop a 20% gain trading system"(we'll call this PURPOSE 1).

Generalizing, why do I want to develop a trading system like this? Because I want to transform it in my major business. So the 2nd purpose can be "I want to PR these books because I want develop a new trading system for becoming financially independent with a business that gain at least 20% per time". (we'll call this PURPOSE 2)

But why do I want a business like this? Because I want to gain financial freedom and consequently my time, for using it in family, in other things I like & so on. So, the ultimate purpose can become "I want to PR these books for developing a trading business, which allow me to use my time for what I love (family, passions, ...) (we'll call this PURPOSE 3)

So, which is the right purpose? I think the more precise is the 2nd, but the 3rd is the ultimate purpose.
Waiting for your answers/suggestions, thank you.
Bye