This chapter explores psychedelics as catalysts of spontaneous thought. Classic serotonergic psychedelics such as psilocybin, LSD, and ayahuasca can induce potent alterations in cognition and perception. The chapter reviews research on these substances through the lens of cultural neurophenomenology, which aims to trace how neurobiology and sociocultural factors interact to shape experience. After a decades-long hiatus, the scientific study of psychedelics is rediscovering the potential of these substances to promote creative insight, evoke mystical experiences, and improve clinical outcomes. Moreover, neuroimaging experiments have begun to unravel the influence of psychedelics on large-scale connectivity networks of the human brain. Tapping perspectives from the social sciences, the chapter underscores how culture and context constrain the flexible cognitive states brought about by psychedelics. This integrative approach suggests that seemingly spontaneous psychedelic thought patterns reflect a complex interaction of biological, cognitive, and cultural factors - from pharmacology and brain function to ritual, belief, and expectation.