The common core thesis contends that mystical experience is an ultimate non-sensuous experience of unity of all things. It can be identified within major faith traditions, whether explicitly religious or not. Its roots are in the work of William James who explored mystical experience outside the limits imposed by what he perceived as only a provisional natural science assumption of the newly emerging discipline of empirical psychology. Following the explicit phenomenological work of Walter Stace, the phenomenology of a universal core to mystical experience has been operationalized and an explicit psychometric measure developed to allow empirical assessment of the claim to a common core to mysticism. It is the linkage of psychometric approaches to the work of James and Stace that is now known explicitly as the common core thesis. The common core thesis needs to be delineated from the perennialist thesis popularized by Aldous Huxley in which there is postulated not only a common core experience, but also values and practices claimed to be associated with this experience if not directly derived from it. Psychometric and empirical evidence for the common core thesis is substantial and continues to accumulate. The common core thesis is restricted to mystical experience and assumes that this experience seeks to express itself in various faith traditions, whether religious or not, but is not restricted to or defined adequately by the culture or language with which this experience is interpreted. Unlike the perennialist thesis, the common core thesis does not assume that any common theology, philosophy, or practice necessarily follows from mystical experience.