Mill defended an entirely naturalistic conception of the subject.
All knowledge ultimately stems from experience.
Even the results of mathematics and logic are known only through empirical verification. The only acceptable form of inference is generalization from repeated experiences. The excesses of utilitarianism are checked by the fact it is in everyone's best interest to set up a society which allows for the greatest liberty for the greatest number. He also argued that political freedom requires economic security therefore the only politically viable state would be a socialist one. Everyone is welcome.