How does Mill define happiness and pleasure in “Utilitarianism? Does Mill refer to these experiences as the same thing?

Critics of utilitarianism contend that virtue is separate from utility or happiness. How does Mill respond to these critics? Does the idea of virtue contradict Mill’s Utility Principle?

Why does Mill believe it is impossible to provide direct proof for the Utility Principle? What is the proof that Mill does provide in Chapter 4 of “Utilitarianism”? How would you critique Mill’s proof?