Users of psychedelic drugs often report that their sense of being a self or ‘I’ distinct from the rest of the world has diminished or altogether dissolved. Neuroscientific study of such ‘ego dissolution’ experiences offers a window onto the nature of self-awareness. We argue that ego dissolution is best explained by an account that explains self-awareness as resulting from the integrated functioning of hierarchical predictive models which posit the existence of a stable and unchanging entity to which representations are bound. Combining recent work on the ‘integrative self' and the phenomenon of self-binding with predictive processing principles yields an explanation of ego dissolution according to which self-representation is a useful Cartesian fiction: an ultimately false representation of a simple and enduring substance to which attributes are bound which serves to integrate and unify cognitive processing across levels and domains. The self-model is not a mere narrative posit, as some have suggested; it has a more robust and ubiquitous cognitive function than that. But this does not mean, as others have claimed, that the self-model has the right attributes to qualify as a self. It performs some of the right kinds of functions, but it is not the right kind of entity. Ego dissolution experiences reveal that the self-model plays an important binding function in cognitive processing, but the self does not exist.

Can the use of psychedelic drugs induce lasting changes in metaphysical beliefs? While it is popularly believed that they can, this question has never been formally tested. Here we exploited a large sample derived from prospective online surveying to determine whether and how beliefs concerning the nature of reality, consciousness, and free-will, change after psychedelic use. Results revealed significant shifts away from ‘physicalist’ or ‘materialist’ views, and towards panpsychism and fatalism, post use. With the exception of fatalism, these changes endured for at least 6 months, and were positively correlated with the extent of past psychedelic-use and improved mental-health outcomes. Path modelling suggested that the belief-shifts were moderated by impressionability at baseline and mediated by perceived emotional synchrony with others during the psychedelic experience. The observed belief-shifts post-psychedelic-use were consolidated by data from an independent controlled clinical trial. Together, these findings imply that psychedelic-use may causally influence metaphysical beliefs—shifting them away from ‘hard materialism’. We discuss whether these apparent effects are contextually independent.

Philosophy of Psychedelics is the first scholarly monograph in English devoted to the philosophical analysis of psychedelic drugs. Its central focus is the apparent conflict between the growing use of psychedelics in psychiatry and the philosophical worldview of naturalism, which holds that the natural world is all that exists. The book reviews scientific evidence that psychedelics such as lysergic acid diethylamide (LSD) and psilocybin can be given safely in controlled conditions, and can cause lasting psychological benefits with one or two administrations. Supervised psychedelic sessions can reduce symptoms of anxiety, depression, and addiction, and improve well-being in healthy volunteers for months or even years. But these benefits seem to be mediated by ‘mystical’ experiences of cosmic consciousness, which prompts a philosophical concern: Do psychedelics cause psychological benefits by inducing false or implausible beliefs about the metaphysical nature of reality? The author integrates empirical evidence and philosophical considerations in the service of a simple conclusion: This ‘Comforting Delusion Objection’ to psychedelic therapy fails. Exotic metaphysical ideas do sometimes come up, but they are not the central driver of change in psychedelic therapy. Psychedelics cause lasting psychological benefits by altering the sense of self and changing how people relate to their minds—not by changing their beliefs about the ultimate nature of reality. The upshot is that a traditional conception of psychedelics as agents of insight and spirituality can be reconciled with naturalism. Controlled psychedelic administration can lead to genuine knowledge gain and spiritual growth, even if no cosmic consciousness or divine Reality exists.

‘On the need for a natural philosophy of psychedelics’ provides a brief overview of the history of psychedelics in science and psychiatry, and a detailed review of recent evidence for their therapeutic and transformative efficacy. The chapter reviews clinical trials showing that one or two controlled psychedelic experiences can durably reduce symptoms of anxiety, depression, and addiction, and can cause lasting psychological benefits in healthy subjects. The chapter also reviews evidence that these beneficial effects are mediated by the occurrence of ‘mystical-type’ experiences. This evidence gives rise to the Comforting Delusion Objection that is the central focus of the book. Existing responses involve (i) rejecting philosophical naturalism, or (ii) downplaying the importance of epistemic factors in the evaluation of psychiatric treatments. The chapter raises problems for both approaches, and outlines a new one: to show that the Objection fails even if naturalism is true and the epistemic status of psychedelic therapy is important.