Psilocybin is one of a class of compounds whose primary activity is known to be on 5-HT-2a/c serotonin receptors. Their effects include changes in perception and cognition. In the pharmacology literature, this class of drugs is called "hallucinogens, " though they rarely cause "hallucinations" in the sense of seeing or hearing things that are not there. Within other academic fields, the term ‘entheogen,’ roughly meaning "spirit-facilitating," is coming into prominence for this class of substances.