Marx deplored political economy's claims to establish “eternal” – or “natural” – laws. This paper seeks to defend John Stuart Mill from his critique. It argues that, contrary to what Marx alleged, these two economists have a great deal more in common on this topic than is frequently realised. Both on the theoretical level and on the political one, Mill's views about the relativity of capitalism seem very close to Marx's. This paper also suggests that Marx may have ignored Mill's insistence on the relativity of economic theories because it may have challenged his own “scientific socialism”.