Increasing evidence suggests that psychedelic-assisted psychotherapy can durably reduce symptoms of anxiety, depression, and addiction. Despite increasing enthusiasm about this treatment modality, including “Breakthrough Therapy” designations and talk of a “new paradigm” in psychiatry, fundamental questions remain unresolved. First, it is unclear whether psychedelic administration has a genuine causal effect on symptomatology, given problems concerning blinding and placebo control. Second, if there is a real causal effect, it is unclear whether it is caused partly by psychedelic experiences themselves, or wholly by non-experiential neurobiological processes. So far, both questions have eluded decisive empirical resolution. Building on recent discussions of these issues, I formulate novel, or at least under-appreciated, arguments that psychedelic treatments do have a genuine causal effect on psychiatric symptoms, and that
psychedelic experiences themselves are part of the causal mechanism underlying this effect. These arguments involve interpreting existing empirical evidence through the lens of philosophical frameworks for thinking about mechanistic explanation, levels of explanation, and mechanistic evidence. I also consider possible experimental avenues for making further progress on these issues.

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Dr Chris Letheby is a philosopher working on issues related to the therapeutic and transformative potential of classic psychedelic drugs. His doctoral research, conducted at the University of Adelaide, presented the first systematic analysis of psychedelic experience within the framework of 21st-century philosophy of cognitive science. He is currently a Lecturer in Philosophy at the University of Western Australia and a Postdoctoral Researcher at the University of Adelaide on the ARC-funded project "Philosophical Perspectives on Psychedelic Psychiatry". His monograph Philosophy of Psychedelics is forthcoming with Oxford University Press.

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