 The general attention to language turned the project of logical empiricism into the problem of the acceptable universal language. Science must be characterized by its proper scientific language. Neurath’s plan was for a unified science against metaphysics, and this involved a plan to police the proper unifying, metaphysics-free language. The negative track of the plan was effectively carried out by the development and adoption of a list of ‘dangerous terms’, which he sometimes referred to, semi-jokingly, as an index verborum prohibitorum: a list of forbidden terms such as ‘I’, ‘ego’, ‘substance’, etc. (Neurath 1933, Neurath 1940/1984, 217). The positive track was the adoption of the universal language of physicalism: ‘metaphysical terms divide—scientific terms unite’ (1935, 23, orig. ital.).