David J. Blacker’s recent Deeper Learning with Psychedelics is a valuable attempt to think through the implications of psychedelics for philosophy and education. One passage in particular caught my imagination: Blacker points out the similarities between a psychedelic experience and René Descartes’s passage of radical doubt.

As Blacker notes, this may be the most famous passage in any of the world’s philosophical texts, but it bears a quick recap. Many have noted Descartes’s social context amid great wars over belief between Catholics and Protestants, wars that called much into question – but the passage itself is not about these. Rather, it is Descartes speaking about a way in which one personally and individually allows oneself to doubt absolutely everything – to imagine that all one’s perceptions could be the work of an evil demon – and comes out recognizing thus that the one thing that cannot be doubted is the doubt itself, for it makes no sense for the doubt not to exist. (Many intro philosophy classes illustrate this exercise of Descartes’s with a clip from The Matrix.) A Buddhist would question Descartes’s next move – where he says the doubt must be the work of a doubter, a self – but we can leave that aside for the moment. The point in the present context is that, according to Descartes, one emerges from the doubting episode not with doubt but with certainty, certainty expressed in those famous words “I think therefore I am”, and out of which one is able to return to the world newly confident in its reality.

What Blacker ingeniously points out is the episodic nature of this doubt. “Cartesian doubt” does not keep showing up in the rest of Descartes’s philosophy. Rather, it is a specific, time-bounded practice in which one suspends belief: one enters doubt for a brief period in order to immunize oneself against it. Descartes says “I realized that it was necessary, once in the course of my life, to demolish everything completely” (VII.17; Blacker rightly adds the emphasis). “Cartesian doubt”, in Descartes’s own view, is not a perspective one should inhabit, it is something kind of crazy – just not completely crazy, which is why one needs to have a brief episode of entertaining it. And in that, the practice bears a significant resemblance to that which one might see on psilocybin or LSD or ayahuasca: this is a temporally bounded activity in which one might temporarily see things that are unreal, yet when one then returns to everyday reality one nevertheless sees it better.

Following Pierre Hadot, Blacker reminds us it’s no coincidence that Descartes entitles this work the Meditations on First Philosophy. Far from the “view from nowhere” with which Descartes is sometimes associated, the Meditations are intensely first-personal. They are spiritual exercises, practices in which one engages to improve oneself. Blacker notices that it seems Descartes did not himself, autobiographically, practise such an exercise – yet he is nevertheless recommending it as spiritually beneficial. In that, he may bear a significant similarity to Buddhaghosa and the author of the Cloud of Unknowing: they do not claim to be describing experiences they have had, but prescribing experiences that one could, and indeed should, have.

The episodic nature of Descartes’s philosophical meditation should be familiar to Buddhists, too. I am particularly struck by its similarity to the ninth, metaphysical chapter in Śāntideva’s main work, the Bodhicaryāvatāra. This chapter always poses challenges to teachers, because where the rest of the book is a largely accessible invitation to Mahāyāna Buddhist ethical practice, the ninth gets deep into the weeds of Śāntideva’s contemporary metaphysical and epistemological debates, just as a contemporary analytic philosopher might. Yet it’s within that analytical chapter that one finds what may be the text’s most highly regarded verse: “When neither existents nor nonexistents stand before the mind, then, because it has no other destination, then the mind, without objects, becomes tranquil.” (IX.34) In the midst of all the technical argument, here we find a description of the mind’s tranquility – a spiritual accomplishment so high that Tibetan historians associate this verse with the exercise of magical powers.

Śāntideva’s description of the mind’s objectless awareness bears a similarity to the nondual mystical passages in Zhuangzi, Gauḍapāda, and Eckhart. But there is a significant difference. What those three thinkers seem to describe is a meditation in the sense we are used to nowadays: they withdraw from everyday perception. From their descriptions, one imagines eyes closed in the states they are describing. But in Śāntideva’s passage, no such withdrawal of perception is mentioned: the mind gets to objectless awareness purely by philosophical argument. And in that – despite all the objections Śāntideva would no doubt have to the Cartesian self – there is a strong similarity to the argument that Descartes himself describes as a meditation.

Now Śāntideva’s phrasing does leave open the possibility that in his view the mind’s objectless tranquility is an enduring, long-term state of character, rather than a transient mystical experience. His passage expresses a skepticism not unlike the one in Descartes’s episode, where everything is in some sense an illusion, but unlike Descartes he does not return to the everyday world by repudiating that skepticism; rather he attempts to harmonize the skepticism with everyday experience through the distinction between conventional and ultimate. Still, the placing of that tranquility in the book – in the uncharacteristically technical chapter wedged between an ethical argument for altruism and a final celebration of redirecting one’s good karma to others’ benefit – does suggest a transient state comparable to the one in Descartes, where one enters a skeptical state and then returns in some respect to conventional reality. It calls to mind the Chan/Zen Oxherding Pictures, where in one’s spiritual progress self and world both disappear – but then return.

In case it needs to be said, none of this is to suggest that Descartes or Śāntideva or any of the other authors were actually using mind-altering substances in their philosophical reflection. I don’t think they were. It’s only to note a structural similarity that often goes unnoticed: the way that philosophical reflection can be a process of leaving and returning to the everyday world, much like many kinds of psychedelic experience – or other mystical experience.