According to Mill, the greatest objection to the Utilitarian or Happiness Theory is the idea of justice. Many argue that if it is moral to achieve personal happiness or common (public) happiness, justice can play no part, as justice considers personal liberty, property or anything granted by law as is most applicable to society above the interests of personal happiness.
Everyone seems to agree that justice is necessary in most, although not all, areas of life. Justice is not something that can be excluded.
Justice works to promote good for everyone. But happiness would seem to promote only one person: the individual experiencing the happiness. Although, this does not take into consideration the idea of sympathy, which states that we feel what others feel.
Mill accepts this argument as a valid one, however, he says that it demonstrates a lack of understanding of the happiness theory itself.
But happiness is found in justice, as justice is defined by the common good, regardless of individual happiness. How can there really be such a thing as individual happiness with the acceptance of the idea of sympathy?
I believe it is this definition (that happiness in not individual but of the public, as each man experiences his neighbor’s emotion) that gives support to Mill’s Happiness Theory. Because happiness is the common good, just as justice is for the public benefit, the argument against the Utilitarian Theory based on inconsistency between the theory and justice is a null argument, void of true validity.