It's possible to see the whole human growth movement of the 1970s as a wish to continue the inward quest without having to put yourself on the line in the way you had to when you took 250 gamma of LSD. And I think all these other methods are efficacious, but I think it's the sheer power of the hallucinogens that puts people off.
Terence McKennaCertainly neo-Platonism, Plotinus and Porphyry and that school are psychedelic philosophers. Their idea of an ascending hierarchy of more and more rarefied states is a sophisticated presentation of the shamanic cosmology, which is the cosmology that one experientially discovers when they involve themselves with psychedelics.
Terence McKenna