First published in the 1970s, this edition has an updated preface that renders the material current. The authors feel that the distribution of voluntary PSI abilities within the general population is far from the bell-shaped or "normal" curve, and that many of the field's strongest experimental outcomes are obtained in a repeatable fashion because high between-subject variation seemed certain in terms of PSI capacity and whatever properties of physiological organisation and function might underlie it. The central strategy advocated was to approach PSI indirectly, through intensive study of various altered states of consciousness with which it appears to have been strongly associated, historically and cross-culturally. These unusual states of consciousness seem more accessible than PSA itself to psychophysiological analysis with a possibility of a solution to the replicability problem.