I was wondering if you might be able to clarify this a little: are you looking for an explanation about Hart's broader point here? Or -- as the title sort of suggests -- more focused on an explanation of the use of the notion of felicity/"happiness" (i.e., the skillfulness of a conceptual distinction), as a part of the technical vocabulary of academic philosophy..?
Yes. I'm "looking for an explanation about Hart's broader point here".
I don't understand why Hart's unhappy about "The expression ‘objective’ and its partner ‘subjective’ because, as far as negligence is concerned, they obscure the real issue".
What's "the real issue"?
How do 'objective' and 'subjective' obscure the real issue?