This article examines the first emergence of theories of advertising in the psychological language of the nonrational mind in Britain. The theories appeared from the close of the nineteenth century in a new genre of advertising literature: books, essays, pamphlets, course offerings, and periodical publications dedicated to advertising. In dialogue with forgotten 1911 novel by Oliver Onions, Good Boy Seldom: A Romance of Advertisement, the analysis considers the anxieties that attended the new theories, which attributed unusual power to advertising and therefore challenged perceptions of the capitalist economy as disenchanted and disenchanting. It also shows the efforts that professional advertisers made to reconcile their theories with views of consumers as rational, and of the advertising industry itself as a rationalizing force. Their efforts suggest a misinterpretation by Onions and critics of advertising that he foreshadowed, who portrayed advertising professionals as bold canvassers of the public psyche. In fact, they were insecure and uncomfortable with their terms of expertise, and developed them because mounting criticisms levelled at advertising left them little choice. Nonetheless, Onions captured the lasting power of this transformation. Despite their insecurity, early professionals created a myth still harbored today, that advertisers are masters of subliminal control in capitalism.