W.T. Stace explained in The Teachings of The Mystics – as has been noted by previous researchers, including Rudolf Otto and Evelyn Underhill – that, “There appear to be two main distinguishable types of mystical experience, both of which may be found in the higher cultures. One may be called extrovertive mystical experience, the other introvertive mystical experience. Both are apprehensions of the One [“The Divine” or “Ultimate Reality”], but they reach it in different ways. The extrovertive way looks outward and through the physical senses into the external world and finds the One there. The introvertive way turns inward, introspectively, and finds the One at the bottom [i.e., the core] of the Self…”
Stace provided another summary of the two types of mystical experience in his work Mysticism and Philosophy: “[T]he extrovertive mystic, using his physical senses, perceives the multiplicity of external material objects – the sea, the sky, the houses, the trees – mystically transfigured so that the One, or the Unity, shines through them. The introvertive mystic, on the contrary, seeks by deliberately shutting off the senses, by obliterating from consciousness the entire multiplicity of sensations, images, and thoughts, to plunge into the depths of his own [being]. There in the darkness and silence…he perceives the One – and is united with it – not as a unity seen through the multiplicity (as in the extrovertive experience) but as the wholly naked “One” devoid of any plurality whatever.”