top of page

"Psychedelics and the Limits of Naturalism": A Review by Peter Sjöstedt-Hughes

White house on hill

Philosophy and psychedelics are both, in the broad sense, mind expanders. The relative infrequency of their convergence is therefore something of a mystery. There are no doubt many factors for this that include twentieth-century legal prohibitions against such chemicals; the propaganda warnings of grave risks to brain, body, and society; the spiritual hegemony of the Church against other forms of metaphysical experience; the consciousness-reductivism (sometimes, -denialism) from the academe in the last century, to mention but a few. We are now, however, in the midst of the so-called “psychedelic renaissance”, spearheaded for the most part by clinical therapeutic uses for psychedelics, so it is time for academic philosophy to step into the light to encounter and analyse the myriad fascinating issues that psychedelics occasion, issues that concern all the major facets of philosophy: philosophy of mind, phenomenology, axiology (ethics and aesthetics), epistemology, metaphysics. Into this encounter is welcomed the particular perspective brought from philosopher Dr Chris Letheby in his book, Philosophy of Psychedelics. The text is a refashioning of Letheby’s doctoral thesis from The University of Adelaide where he is, at the time of writing, a postdoctoral researcher. He is also a lecturer at The University of Western Australia; and he follows, to speak generally, the line of Australian materialism.

Before critically engaging with the monograph, I will offer a summary of its suppositions, purposes, and arguments. What first strikes the reader as particularly impressive is the clarity of the detailed analyses that Letheby provides. Here is a sharp mind approaching an ambiguous mist – a complex interwoven plasma of perplexities surrounding consciousness, conditions of consciousness, novel varieties of consciousness, all of which tie to issues involving therapy, society, and cosmos. The second noteworthy quality is the contemporaneity of the book, reflected in the abundance of references to, and analyses of, the latest scientific research on clinical psychedelic trials. Here we glimpse the clash that the book addresses – the meeting of the mystical with the scientific.


Psychedelics can induce, it is reported, “mystical” experiences. Walter Stace, in his 1960 tome Mysticism and Philosophy, with influence from psychedelic philosophy pioneer William James (surprisingly not referenced in Letheby’s book), identifies such experiences as those of “unity”, of timelessness and spacelessness, a noetic feeling of reality, a sense of the sacred, of the divine, states of apparent paradoxicality, and further states defying description, reportedly ineffable. The key issue that Letheby considers in this book relates to the role of such mystical experience in psychedelic therapy – given naturalism. For Letheby’s philosophical commitment is to naturalism/physicalism, which takes the fundamental substance of reality to be physical, as that word is understood by the natural sciences. Under this metaphysical theory, consciousness can only be a product of – or something identical or reducible to – the physical, or more specifically the neuro-physical. Moreover, such naturalism rejects any transcendent realities, divine or otherwise.

It seems that the main therapeutic mechanism of psychedelics is the induction of a non-naturalistic, mystical experience.

The application of naturalism to psychedelic therapy gives rise to an apparent problem: the so-called “Comforting Delusion Objection” (CDO). It seems that the main therapeutic mechanism of psychedelics is the induction of a non-naturalistic, mystical experience: transcendent realities, the fundamentality of mind over or parallel to matter, and so on. Given naturalism, however, these experiences must be delusions – albeit comforting, therapeutically effective ones. But is it not unethical to treat mental health problems by imposing a falsity? As Letheby puts it, “there seems to be something seriously questionable about a treatment … that works by inducing metaphysical hallucinations” (p. 2).

The Comforting Delusion Objection is an idea put forward by the writer Michael Pollan in his 2015 essay “The Trip Treatment” from which Letheby extracts the following quotation: “Is psychedelic therapy simply foisting a comforting delusion on the sick and dying?” Letheby goes through the three common responses to the objection. The first response is that mystical psychedelic experience is not a delusion but veridical. The psychologist Benny Shanon is referenced who is known for his systematic study of the phenomenology of the psychedelic Amerindian brew, ayahuasca. Shanon generalizes an overall belief induced by ayahuasca as “idealistic monism with pantheistic overtones”. This is metaphysical parlance for the kind of states that are definitively non-naturalistic and non-physicalist. Shanon himself identifies this with a type of Spinozism. Letheby, however, rejects any metaphysical belief system other than naturalism, and consequently this first response to the objection will not do.

The second response to the CDO is that “it is not so important whether psychedelic-induced mystical experiences are veridical or not” (p. 29). Rather, the important question is whether the psychedelic therapy produces beneficial outcomes in patients. This more pragmatic response can in principle be taken by those of any metaphysical persuasion. But, as an ardent naturalist, Letheby cannot accept the second response either because he does consider the epistemic credentials, the question of the reality of the psychedelic experience, to be important.

The third response to the CDO is an ethical one resulting from the first response: if psychedelic therapy is based on the mechanism of inculcating non-naturalistic beliefs/delusions then it should be prohibited. It is unethical, immoral, as it helps people only by fostering falsities. It is this third response that is of the essence of the CDO – it is the objection to psychedelic therapy rather than – as with the other responses – its defence.

The novel contribution Letheby now introduces rejects all three responses yet still maintains naturalism. As we have seen, he rejects the first response as he assumes naturalism. He rejects the second response as he believes the epistemic status, the veridicality question, of psychedelic experience is important both in itself and in its therapeutic application. But now he also rejects the third response for two primary reasons. The first is his “conjecture” that psychedelic therapy does not actually work by fostering false beliefs or comforting delusions. Rather, psychedelics induce experiences – notably those of “connection, aspiration, and asking the Big Questions” (p.197) – and it these experiences rather than new beliefs, that are the prime mechanism of psychedelic therapy. The second reason is the claim that psychedelic therapy is, borrowing a term from Lisa Bortolotti, “epistemically innocent”. This more positive angle on psychedelic therapy means that despite the “epistemic risks” that psychedelics carry (they can imbue false, non-naturalistic beliefs), they also carry “epistemic benefits” that cannot be reached in other ways. Thus, the CDO against psychedelic therapy fails because the therapy works due to truths, not delusions – even given naturalism.


Walden Pond in November

What, then, are these truths, these epistemic benefits, that psychedelics bring forth? One of Letheby’s core claims is that psychedelics yield novel psychological insights that can be of great therapeutic importance. Patients may experience themselves and their prosaic mental states of anxiety, depression, addiction, and so on, in a grander scheme of reality where negative objects of thought are rendered relatively unimportant. One shifts from damaging forms of self-consciousness to states of apparent “cosmic consciousness”, for instance (one example of the platitudinous “ego-loss” so often reported by psychonauts). Such experiences are often interwoven with feelings of connection to others and Nature, and so any sense of isolation or loneliness may diminish. Letheby’s acknowledged conjecture is that such shifts in one’s psychological attitudes to self, others, and cosmos are not necessarily delusions but rather alternate, epistemically beneficial varieties of looking at oneself in relation to the environment. It is not a delusion to re-appraise the importance of objects of thought that have hitherto caused one anguish. Such psychedelic-induced re-appraisals of self, Letheby proposes, need not occur via a change in metaphysical belief (as the CDO would have it); rather they can occur by simply having such experiences, and thus realizing that other, more positive modes of mind are possible.

However, it is not only experience rather than belief that is causally effective in the book’s speculations, but also experience rather than pure physiology. Letheby argues that it is the experience qua experience that plays the key causal role in this psychedelic therapy, as opposed to purely physiological, neurological changes affected by psychedelic compounds. The proposition that it is the experience rather than the pure physiology that does the therapeutic work may appear obvious to many readers, but there is current research on developing therapeutic “psychedelic” compounds that occasion no psychedelic experience. What is of philosophical interest here is Letheby’s implicit rejection of epiphenomenalism (the idea that a mental state or experience has no causal power). As we will come to see, this rejection is not easily made compatible with the naturalism Letheby advances. As a naturalist, with respect to theories of experience, Letheby must accept that a necessary and sufficient condition for consciousness is a physical, generally neurophysiological, base. He puts forward a rather bold neurocognitive theory to not only substantiate his naturalist claim that there cannot be any kind of “cosmic consciousness”, or any dualistic spirits, but also to augment the aforementioned therapeutic epistemic benefits from psychedelics with philosophic epistemic benefits, especially with regard to our concepts of self and time. Let us then look at the theory of mind that explicitly underlies the book’s conjectures and arguments.

Letheby advocates a “computational theory of mind … which holds that the mind is a computational system implemented in the ‘wetware’ of the biological brain” (p. 102) where “conscious experiences … are nothing other than complexes of neurally implemented mental representations, all of which bear some consciousness-making property” (p. 104). A “consciousness-making property” – when I read that line I could not help but think of a passage in Nietzsche’s Beyond Good and Evil where he refers to an answer given by a doctor in a Molière play: “How can opium make us sleep? It is ‘facilitated by a faculty’, the virtus dormitiva [sleeping virtue]”. Does the naturalist explanation of consciousness essentially stop at this unknown, mystical consciousness-making faculty? There are innumerable problems with computational theories of mind, especially regarding problems of reduction, but what is most pertinent to the purpose of this book is the relation that such a naturalistic theory of mind has with the mystical experiences that psychedelics can evoke.

To claim that conscious experience is exclusively neurally implemented by a “consciousness-making property” is not scientific but is, in fact, itself a mystical claim.

Letheby acknowledges that his theory “does not constitute a solution to the Hard Problem of Consciousness: that remains unsolved” (p. 105). The Hard Problem of Consciousness, the question as to the relation between mind and matter, is crucial to determining the veridicality or delusiveness of psychedelic-induced experiences of not only cosmopsychism – an overmind, a “cosmic consciousness”, or (in the words of Spinoza) an “infinite intellect” – but also of the possibility of minds in nature (panpsychism) related to the experiences of nature-connectedness that psychedelics can invoke, as well as to the animistic experiences that Amerindian cultures associate with their longstanding psychedelic use. If we do not know the solution to the Hard Problem of Consciousness, then we do not know that the brain, or “wetware”, is a necessary and sufficient condition for consciousness. Neural correlates of consciousness merely present the problem rather than the solution. To claim that conscious experience is exclusively neurally implemented by a “consciousness-making property” is not scientific but is, in fact, itself a mystical claim. The mind-matter relation is a mystery, thus the “Hard Problem”. And there are clear problems in rejecting a mystical experience as delusional through the use of a mystical premise. Such a move is akin to using flat-earth theory to reject astrology as unscientific. The point is not that such a computational theory of mind is wrong, but rather that the mystery as to how it could work should keep us more open to other metaphysical frameworks that might be suggested by psychedelic intake.

David Chalmers, who coined the “Hard Problem” made this general point himself from the start: “[A] solution to this problem may profoundly affect our conception of the universe and of ourselves.” The Hard Problem of Consciousness is directly related to the determination of metaphysical frameworks. Of course, metaphysical naturalism need not take a computational theory of mind (there are many other possibilities for it); nonetheless, all of them halt at the Hard Problem of Consciousness. Naturalism is itself an epistemic risk, though not necessarily a comforting delusion.


Letheby is brave in the face of such risks and delves yet deeper down the rabbit hole. In Chapter 6, “Resetting the Brain”, he augments his mind theory with the rather controversial predictive processing theory of cognition according to which “conscious experience is a ‘controlled hallucination’ – an internal virtual reality or world simulation” (p. 111). The brain predicts its environment, and the role of external perception is merely to control mispredictions. It is of note that we have travelled through naturalism here to what appears to be a new form of idealism, a techno-solipsism. In fact, the basis of the predictive processing theory is Kantian transcendental idealism, a theory that Letheby considers, somewhat paradoxically, non-naturalistic. Like Kantian idealism, in the predictive processing metaphysical framework not only is the world of colour, sound, smell, etc., a mere representation of reality, but so are other more fundamental categories and pure intuitions such as space, time, and causality. Letheby fascinatingly speculates that psychedelic chemicals disintegrate normal brain functioning and thereby “reduce the brain’s confidence in these fundamental beliefs” (p. 119), and it is this that triggers the commonly reported experiences of the transcendence of time and space.

Letheby, however, avoids what seems to be the obvious next step, which is to question, à la Kant, whether external reality actually is spatiotemporal in the sense in which we represent it, considering that spatio-temporality is a subjective representation. For Kant, this question resulted in the bifurcation of reality into our phenomenal world and the actual, mind-independent noumenal reality to which we have no cognitive access. Letheby does not make this move, however, as it would undermine the physicalism that started off the enquiry. Yet the issue still stands: if the reality we perceive is a “controlled hallucination”, then why shouldn’t the physical itself be considered a mere hallucination? If it is, then the whole naturalist theory implodes. There may be a way out of this rabbit hole, but as it is, the theory seems more abstractly metaphysical and mystical than some of the psychedelic states it is supposed to judge as delusional.

It might also be argued that the “controlled hallucination” theory of mind is incompatible with the sense of connectedness that features very prominently in the later sections on “Naturalistic Spirituality”. The experience of connectedness to others, to Nature, to oneself, is, Letheby argues, a crucial feeling with immense therapeutic value – albeit without the need for any non-naturalistic metaphysical belief. At the same time, such a feeling of connectedness is deemed a hallucination via the predictive processing theory that underlies the project of naturalising spirituality. For instance, Letheby writes: “All the furniture of our waking experience – the people, animals, plants, tables, chairs, and our own bodies and selves – is as thoroughly virtual, internally constructed, and simulatory as the fantastic creations of nocturnal dreams and psychotic hallucinations” (p. 115). Could there be anything more disconnecting than such a belief? If true, it would mean that we never experience real connection to our children, our families, the natural world, nor to ourselves (the last of which is but an “empirical apperception”, as Kant would call it). To overcome such alienation, such disconnection, one must stop assuming, as does Kant and the techno-solipsists, representationalism and adopt instead a perceptual realism, in the vein of Alfred North Whitehead. In this alternate metaphysic, the relation of subject and object is not one of representation-to-represented but of part-to-whole: the perception becomes part of the perceiver – a veridical rather than hallucinatory connectedness. But this debate lies elsewhere.

A further problem faced by Letheby is how naturalism approaches the normative ethical questions relating to the Comforting Delusion Objection to psychedelic therapy. The idea of “epistemic risks” and the claim that fostering delusions is unethical requires a standard of ethics to which such judgements are made. However, the common naturalistic approach to ethics is Moral Anti-Realism: that there exist no transcendent, objective moral ideals or standards, but only subjective, culturally-inculcated ideals. The naturalist is more Thrasymachus than Socrates in this respect. Thus if – and this can of course be debated – naturalism implies Moral Anti-Realism, the Comforting Delusion Objection fails on that account alone and requires no fourth way: inculcating delusions cannot be immoral (contra the third response), and treating patients cannot be moral (contra the second response). Furthermore, as concerns the fourth response, the upholding of truth itself – “epistemic benefits” – becomes less important in light of Moral Anti-Realism as it cannot be an ethical imperative. The Comforting Delusion Objection is a moral objection, yet naturalism implies amoralism at the fundamental level.

As well as such theoretical challenges, there is a more empirical one to consider. It was an acknowledged conjecture that the core mechanism for psychedelic therapy was not a shift in metaphysical beliefs but rather the experience itself sans belief change. Though I believe this has elements of truth, the conjecture has been weakened a little by empirical research published after the book’s release that suggests the opposite: there actually seems to be a tendency for psychedelics to shift takers’ metaphysical beliefs from that of physicalism to that of panpsychism. Letheby’s book is impressively contemporary as regards recent scientific studies, yet the danger of a book being up-to-date is that it will soon be out-of-date. However, Letheby is perfectly cognisant of the risks of relying too heavily on the latest empirical evidence, and it is certain that many of his arguments – especially the theoretical ones that are less subject to the whims of the empirical world – will stand the test of time.


Rather than criticisms, one can see most of these issues as cues for debate on issues at the most fundamental philosophic levels. I personally endorse a more neutral monistic metaphysic than Letheby and thus points of disparity are inevitable. I would have liked to see more discussion differentiating types of metaphysics, including non-Western types, rather than the simplistic dichotomy of naturalism/non-naturalism. Presenting an array rather than dichotomy of such metaphysical views would have differentiated the possibilities of veridicality and delusion in a way that is relevant to the CDO: for instance, under a Spinozist metaphysic, experiences of an “infinite intellect” would be considered potentially veridical whereas experiences of disembodied spirits would be considered hallucinatory. Determining “delusion” requires a determination of reality. Naturalism is only one such proposed determination and there are multiple others – many of which have no doubt not yet been conceptualized.

One of the important contributions that psychedelics bring is to offer a person novel conceptualizations of reality.

One of the important contributions that psychedelics bring is to offer a person novel conceptualizations of reality. The late Swedish author and psychonaut Patrick Lundborg pleaded that one should let the psychedelic experience provide one with a metaphysics rather than letting a metaphysics interpret the psychedelic experience. This is a worthy ideal, albeit one that is very difficult to apply. But it does suggest again the role and value that philosophy should play in analysing psychedelic states. What Chris Letheby has given the world is the first academic philosophic monograph on psychedelics that combines rigorous scholarship in both philosophy and cognitive science with an analytic mind that thereby clarifies certain contemporary issues in the psychedelic renaissance with fascinating, original ideas – even if some of them are a bit “out there”. But what else would one expect and hope from a book on philosophy and psychedelics?

Philosophy of Psychedelics by Chris Letheby is published by Oxford University Press.

Peter Sjöstedt-Hughes is philosopher of mind and metaphysics who specialises in the thought of Spinoza, Nietzsche, Bergson, and Whitehead, and in fields pertaining to altered and panpsychological states of consciousness. He is a research fellow and associate lecturer at the University of Exeter where he has co-founded the Philosophy of Psychedelics Exeter Research Group, the ambit of which includes taught modules, conferences, workshops, and publications. Peter is the author of Noumenautics (2015) and Modes of Sentience (2021), co-editor of Philosophy and Psychedelics (forthcoming), the TEDx Talker on “Consciousness and psychedelics”, and he is inspiration to the inhuman philosopher Marvel Superhero, Karnak.


From The Philosopher, vol. 110, no. 2 ("The New Basics: Society"). If you enjoyed reading this, please consider buying a copy of this issue or making a donation. We are unfunded and your support is greatly appreciated.


bottom of page