Bion's central thesis in this volume is that for the study of people, whether individually or in groups, a cardinal requisite is accurate observation, accompanied by accurate appreciation and formulation of the observations so made. The study represents a further development of a theme introduced in the author's earlier works, particularly in Elements of Psychoanalysis (1963) and Transformations (1965). Bion's concern with the subject stems directly from his psycho-analytic experience and reflects his endeavor to overcome, in a scientific frame of reference, the immense difficulty of observing, assessing, and communicating non-sensuous experience. Here, he lays emphasis on he overriding importance of attending to the realities of mental phenomena as they manifest themselves in the individual or group under study. In influences that interpose themselves between the observer and the subject of his scrutiny giving rise to opacity, are examined, together with ways of controlling them.