A vast variety of experiences can be occasioned by psychedelic substances, from the mundane or irritating, to the sublime, world-destroying, world-creating. Under the influence we seemingly encounter alien states of mind, and alien worlds – the mysterious subjective and the mystery of the objective: can psychedelic states reveal any objective reality or are they always subjective? For some explorers, there is a 'noetic' aspect to certain psychedelic states: a feeling that one has undergone, as William James puts it, 'states of insight into depths of truth unplumbed by the discursive intellect. They are illuminations, revelations'. The feeling of obtaining novel knowledge concerning fundamental reality is often accompanied by correlated qualms. In the words of Nobel laureate Octavio Paz, 'The person who takes a [psychedelic] drug implicitly doubts the solidity of reality – he is not sure that it is what it appears to be and what our instruments define it as being, or he suspects that another reality exists.'

For some psychonauts, such noetic feeling suffices for full-on belief in the objective existence (the veridicality) of the apparent realities perceived. Such apparent realities may involve fearful four-dimensional praying mantis machines, fiendish pixies pursuing some vital interstellar factory logistics, or one's becoming a sentient polyhedron, eternally spinning for the sake of love. William James, however, reserves noeticism more for the grander metaphysical schemes such as belief in the unreality of time or space, the unity of subject and its object, moreover the ultimate unity of all things, the ubiquity of minds in all entities or mind in all entities (panpsychism and pantheism, respectively), and an intuition of intrinsic values embedded within nature.

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"Under the influence we seemingly encounter alien states of mind, and alien worlds – the mysterious subjective and the mystery of the objective."

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For others, such so-called psychedelically induced mysticism yields no insight into reality at all. Such rejection comes in two main, albeit contrary, strands: the theist and the physicalist. Certain theistic thinkers consider induced mysticism to be, as it were, fake mysticism, revealing no truth in comparison to the revelations of the ordained saints and established mystics. On the other side, the physicalist, or materialist, also takes induced mysticism to be non-revelatory and thus merely subjective. That which the physicalist and the theistic, religious sceptic share is the belief that the existence of a physical substance (the drug) and its neurological ramifications (the neural correlates of psychedelic consciousness) is a sufficient condition for explaining the psychedelic experience. Here there is no need to involve the divine or the metaphysical as a cause of a psychedelic experience. Thus the experience is either dismissed as sacrilegious and delusional, or as merely delusional.

But beyond these two extremes – the all-out belief of the mantis-veridicalists and the all-out rejection of the theist-physicalists – there lies the possibility that certain experiences are veridical, others non-veridical: some revelations, others hallucinations. How could this veridicality be determined?