I entitled the paper ‘An Essay in Speculative Mysticism’ because it undertakes, in the tradition of such ancient and mediaeval mystics as Plotinus, Pseudo-Dionysius, Hugh and Richard of St Victor, Nicholas of Cusa, Ruysbroeck, and Meister Eckhart, to mate psychological introspection with ontological speculation, focusing on the rather fundamental themes of Identity, Alterity, Transcendant Identity, and Illusion. I acknowledge my more recent, general indebtment to the rich reservoir of contemporary research in the area of Transpersonal Psychology, a research amply documented in the journal of Transpersonal Psychology, as well as in the bibliographies, most notably provided among others, in the respective works of Charles Tart, John White, and Ken Wilber. Since Transpersonal Psychology embraces within its attempts at controlled studies such sundries as altered states of consciousness, parapsychological phenomena, yoga, as well as doctrines which skirt the very frontiers of psychology's experimental ingenuity, namely the tenets of G. I. Gurdjieff, P. D. Ouspensky, H. P. Blavatsky, Oscar Icazo, John Lilly, Roberto Assagioli, and many others, the rapport between a speculative study in mysticism and the background data supplied by Transpersonal Psychology becomes readily apparent. Furthermore, I accede to any reader's deft detection in my essay of a rather uncommon medley of remnants from Thomism, German Romanticism, British Neo-Hegelianism, Vitalism, and Structuralism. If, at last, I were to be questioned on the methodology of my paper, I would have to respond that it probably resembles most closely the methodology of two French Reflective thinkers, viz., Jean Nabert and Henry Duméry, and the French existential metaphysician, Jean Wahl. Since I owe so much to so many, I simply acknowledge my general indebtedness at the beginning and allow the essay to emerge with its own internal cogency. The paper's rationale should, therefore, emanate from a clear inner coherence of its ideas as these reflect communicable psychological experience and unfold some of the latter's ontological implications.