There is a fine line between publicity and advertising. While anyone can see the difference between a full-page ad promoting a soft drink and a two-line classified offering part-time work, it is easy to slide from one mode into the other. This slippage between simple intermediation and a full-scale advertising campaign is what hampers thinking about the question. The classical theory underpinning the function of promotion is that of imperfect competition, or ‘non-price competition’, formulated in 1933 by the post-Keynesian economist Joan Robinson and, quite independently, by Edward Chamberlin of the Harvard School. Non-price competition occurs when, in conditions of oligopoly, the businesses concerned avoid a price war for the sake of their own profit margins, instead competing by promoting the products themselves, in particular by special offers. The increased promotional costs remain advantageous compared with the predictable alternative: loss of income through competitive price-cutting.

As to how effective advertising is in increasing sales, whether it is useful in the short or the long term, and how successful it may be in a contest with other promotional campaigns (‘voice share’ rather than ‘market share’): these questions have been under discussion for a century, with no conclusion in sight. The one thing certain, if we borrow a metaphor from physics, is that advertising is the channel through which money is transferred from industry and finance to the mass media: it is the ‘black box’ that takes in money and emits information. Advertising is the main instrument by which, especially in the twentieth century, the dominant economic interests have financed information. This link between money and information has appeared unbreakable, even in the most radical conception of press freedom: modernity has not succeeded in conceiving a means of creating pluralism in information that is not based on a plurality of economic powers. Hence the difficulty of conceiving pluralism of information in a regime of collective property.