In a public, as we may understand the term, (1) virtually as many people express opinions as receive them, (2) Public communications are so organised that there is a chance immediately and effectively to answer back any opinion expressed in public. Opinion formed by such discussion (3) readily finds an outlet in effective action, even against - if necessary - the prevailing system of authority. And (4) authoritative institutions do not penetrate the public, which is thus more or less autonomous in its operations.-In a mass, (1) far fewer people express opinions than receive them; for the community of publics becomes an abstract collection of individuals who receive impressions from the mass media. (2) The communications that prevail are so organised that it is difficult or impossible for the individual to answer back immediately or with any effect. (3) The realisation of opinion in action is controlled by authorities who organise and control the channels of such action. (4) The mass has no autonomy from institutions; on the contrary, agents of authorised institutions penetrate this mass, reducing any autonomy it may have in the formation of opinion by discussion.