So much has been written about the harm principle central to John Stuart Mill’s classic work On Liberty that any attempt to supplement seems superfluous. However, an anomaly in accounts of one aspect of the text requires rectifying. In this lecture, Greg Claeys emphasises the Malthusian context of Mill’s treatment of marriage, and argues that in On Liberty Mill regards the family, not the individual, as the foundational unit in society, and the right to bear children as conditional upon the recognition of the basic duty to maintain them. His Malthusian proposal to restrict this right is the strongest instance of his application of paternalism to adults in a civilised society, but is in Mill’s view entirely commensurate with the principle of liberty.