Jung and Bion both developed theoretical concepts propounding a deeply unknowable area of the psyche in which body and mind are undifferentiated and the individual has no distinct identity, from which a differentiated consciousness arises. In Jung's case, this is enshrined in his psychoid concept and the associated notion of synchronicity and, in Bion's case, in his proto-mental concept and his ideas on group dynamics. It is by means of these two concepts that Jung and Bion approach and locate a combined body-mind, a monism, in which body and mind are seen as different aspects of the same thing. This paper reviews the claim that although the two concepts are associated clinically with very different situations, their commonality may arise from a similar intellectual basis: both men appear to have been influenced by the same source of vitalist ideas in philosophy including Henri Bergson, and Jung's ideas also exerted a direct influence on Bion.