In this dissertation, I focus on the Plotinian henosis , which is defined as a unitive experience with the One. I argue that Plotinus attained his first henosis , expressed as "a happy pull of Athene" unexpectedly when he was twenty-eight years old. So he started studying philosophy to understand it. In the process, he developed a mystical philosophy that is individualistic. For my argument, I reconstruct the Plotinian Henosis based on the textual analysis of the Enneads . Based on the phenomenological reconstruction, I argue that many aspects of Plotinus' life, such as the characteristics of his community, the style of his lectures and writing, his indifference to traditional religion and religious practices, his free spirit and courage of inquiry, and his emphasis on wonder and perplexity concerning the One can be understood better. I claim that these characteristics also make us rethink the relationship between religious experience and its interpretation. Especially, an experiential reading of the Enneads forces us to consider that the Plotinian henosis might function as a valid epistemological ground that can weaken the claims of complete constructivism. In addition, the modern research of religious experiences reveals a close affinity between the Plotinian concept of the human soul ( psyche ) and the extended concept of consciousness, which is claimed by various schools of modern psychology such as Jamesian psychology, depth psychology, and transpersonal psychology. In this regard, the Plotinian henosis can be fruitfully analyzed as an altered state of consciousness (ASC). Finally, for this reason, I argue that Plotinus developed an early form of an "unchurched mysticism" or a "religion of no religion." Thus we can find the seeds of a radical spiritual or mystical individualism in Plotinus, which he succinctly presented as "the flight of the alone to the Alone." The seeds will later develop in the history of Western mysticism and esotericism into more and more individualist expression. Furthermore, his experiential individualism outside of traditional and institutional religions helps explain for me his popularity in the present as well as in the past.