As reliable facilitators of characteristically unique altered states of consciousness that are notoriously difficult to comprehend, psychedelics naturally attract a multidisciplinary scope for their research, with direct relevance to neuroscience, pharmacology, psychology, as well as philosophy of mind, metaphysics, and epistemology — disciplines united by the common inquiry into the nature of conscious experience. Accordingly, the central focus of this paper concerns the metaphysical debate between internalism and externalism about whether psychedelic phenomenology — in particular, its ‘breakthrough’ level — could be said to directly ontologically depend on external factors, as well as the interpretive implications arising from each of those positions. The main contributions of the author of this paper include the provision of a coherent account of the essential phenomenal characteristics of psychedelic experiences, the integration of relatively fragmented philosophical debates on psychedelic phenomenology into the traditional internalist-externalist framework in metaphysics, as well as the exploration of the extent to which the externalist position on psychedelic phenomenology — traditionally associated with non-naturalism — is compatible with naturalistic explanatory frameworks. In Part 1, psychedelic phenomenology will be outlined first, after which the central inquiries of this paper will be contextualised and qualified. In Part 2, the internalist position on psychedelic phenomenology — both its strong and weak versions — will be discussed. In Part 3, the externalist position will be explored, which will be followed by a final evalutation between the three positions presented in this paper. The paper will conclude in favour of the weak internalist position, and that whilst the externalist position remains significantly more speculative than both the strong and the weak internalist positions, the externalist position cannot be rejected altogether until further progress in consciousness research is made.