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.'