Psychosynthesis is a transpersonal psychotherapy. It was founded by Dr Roberto Assagioli, an Italian psychiatrist who lived from 1888 to 1974. He was involved in some of the early psychodynamic activity early in the twentieth century, but split from Freud at about the same time as Jung. Psychosynthesis was developed between 1910 and the 1950s in Florence and Rome, but in the 1960s became more internationally known with centres opening round the world. This study is an investigation of the ideas lying behind psychosynthesis: these ideas spring partly from scientific study of the unconscious, but they also originate in the long mystical tradition of both the Eastern and the Western world. In tracing back these ideas to their sources, the nature of the knowledge underlying a modern spiritual, or transpersonal, psychotherapy is inevitably discussed. Roots of such a discipline lie in a split tradition within the Western world - psychology aspires to be scientific, religion or mystical knowledge is studied within the discipline of theology, and the two are very little related in our present conception of knowledge. Roberto Assagioli's framework is thus a 'synthesis' in several senses: in the attempt to relate the soul and theology to the personality and psychology: in the attempt to perceive personal developmental patterns as a microcosm of larger social and historical patterns: and in the particular characteristics of his therapy with the individual. The meaning of these syntheses is examined within the context of the knowledge on which he explicitly and implicitly drew. Psychosynthesis is a product of the twentieth century. It originated at the turn of the century when many new ideas were questioning the old certainties of nineteenth century thought. It began to flourish at the time in the 60s when once again criticism was being levelled at the direction of Western development. An examination of its origin and development throws light on many aspects of our present values.