No socialist in the ‘advanced’ world in the third quarter of the twentieth century, whatever his affiliation, can avoid seeing that the actual condition, public reputation and future prospects of socialism are all overshadowed by its tragic twentieth-century history — by the comparative failure of its two major organised traditions, orthodox communism and social democracy — to realise the hopes and aims that were placed in them a hundred, or even fifty years ago. For many of us, indeed, that failure is so complete and decisive that we remain obstinately unattached to either of those two dominant traditions, suspended in the active but confused limbo of small groups, transient protest movements and multiplying ‘tendencies’. Inevitably, the whole scene continues to be dominated, in part at least, by the effort to discover ‘what went wrong’; and the confusion will continue so long as no widely agreed answers to that question have been arrived at. In this essay I want to suggest one direction in which I believe a part of the answer lies — in the generally damaging impact of nineteenth-century Positivism on socialist thought and action.