“Mysticism and Philosophy”, one of Stace's key questions is whether there are a set of common characteristics to all mystical experiences. Based on the study of religious texts, which he took as phenomenological descriptions of personal experiences, and excluding occult phenomena, visions, and voices, Stace distinguished two types of mystical experience, namely extrovertive and introvertive mysticism. He describes extrovertive mysticism as an experience of unity within the world, whereas introvertive mysticism is "an experience of unity devoid of perceptual objects; it is literally an experience of 'no-thing-ness'". The unity in extrovertive mysticism is with the totality of objects of perception. While perception stays continuous, “unity shines through the same world”; the unity in introvertive mysticism is with a pure consciousness, devoid of objects of perception, “pure unitary consciousness, wherein awareness of the world and of multiplicity is completely obliterated.” According to Stace such experiences are nonsensical and nonintellectual, under a total “suppression of the whole empirical content.” Stace finally argues that there is a set of seven common characteristics for each type of mystical experience, with many of them overlapping between the two types. Stace furthermore argues that extrovertive mystical experiences are on a lower level than introvertive mystical experiences. Stace's categories of "introvertive mysticism" and "extrovertive mysticism" are derived from Rudolf Otto's "mysticism of introspection" and "unifying vision". William Wainwright distinguishes four different kinds of extrovert mystical experience, and two kinds of introvert mystical experience: Extrovert: experiencing the unity of nature; experiencing nature as a living presence; experiencing all nature-phenomena as part of an eternal now; the "unconstructed experience" of Buddhism. Introvert: pure empty consciousness; the "mutual love" of theistic experiences. Richard Jones, following William Wainwright, elaborated on the distinction, showing different types of experiences in each category: Extrovertive experiences: the sense of connectedness (“unity”) of oneself with nature, with a loss of a sense of boundaries within nature; the luminous glow to nature of “nature mysticism”; the presence of God immanent in nature outside of time shining through nature of “cosmic consciousness”; the lack of separate, self-existing entities of mindfulness states. Introvertive experiences: theistic experiences of connectedness or identity with God in mutual love; nonpersonal differentiated experiences; the depth-mystical experience empty of all differentiable content. Following Stace's lead, Ralph Hood developed the "Mysticism scale." According to Hood, the introvertive mystical experience may be a common core to mysticism independent of both culture and person, forming the basis of