 In these and other cases, it is important to bear in mind that the arguments in "On Liberty" are grounded on the principle of "utility", and not on appeals to natural rights.Thus, his concept of the slavery into which individuals may not sell themselves would apply to the simple, brutal form of complete unfreedom whose abolition was a burning political issue in his day.But it would not extend to the more extended contemporary notions of slavery by economic means or 'wage slavery'.Not every interpretation or inference requires a source in published work.