A hallucinogen is a psychoactive agent which can cause hallucinations, perceptual anomalies, and other substantial subjective changes in thoughts, emotion, and consciousness. The common types of hallucinogens are psychedelics, dissociatives and deliriants. Although hallucinations are a common symptom of amphetamine psychosis, amphetamines are not considered hallucinogens as they are not a primary effect of the drugs themselves. While hallucinations can occur when abusing stimulants, the nature of stimulant psychosis is not unlike delirium. A debate persists on criteria which would easily differentiate a substance which is 'psychedelic' from one 'hallucinogenic'. Sir Thomas Browne in 1646 coined the term 'hallucination' from the Latin word 'alucinari' meaning 'to wander in the mind'. A 'hallucinogen' and a 'psychedelic' may refer correctly to the same substance. Psychedelics are considered by many to be the 'traditional' or 'classical hallucinogens'. A 'hallucinogen' in this sense broadly refers to any substance which causes changes in perception or hallucinations, while psychedelics also carry a connotation of psychedelic culture. The word psychedelic (From Ancient Greek ψυχή (psychê) mind, soul + δηλος (dêlos) manifest, reveal + -ic) was coined to express the idea of a drug that makes manifest a hidden but real aspect of the mind. It is commonly applied to any drug with perception-altering effects such as LSD and other ergotamine derivatives, DMT and other tryptamines including the alkaloids of Psilocybe spp., mescaline and other phenethylamines. The term 'psychedelic' is applied somewhat interchangeably with 'psychotomimetic' and 'hallucinogen', The classical hallucinogens are considered to be the representative psychedelics and LSD is generally considered the prototypical psychedelic. In order to refer to the LSD-like psychedelics, scientific authors have used the term 'classical hallucinogen' in the sense defined by Glennon (1999): 'The classical hallucinogens are agents that meet Hollister's original definition, but are also agents that: (a) bind at 5-HT2 serotonin receptors, and (b) are recognized by animals trained to discriminate 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) from vehicle. Otherwise, when the term 'psychedelic' is used to refer only to the LSD-like psychedelics (a.k.a. the classical hallucinogens), authors explicitly point that they intend 'psychedelic' to be understood according to this more restrictive interpretation (e.g. see Nichols, 2004). One explanatory model for the experiences provoked by psychedelics is the 'reducing valve' concept, first articulated in Aldous Huxley's book The Doors of Perception. In this view, the drugs disable the brain's 'filtering' ability to selectively prevent certain perceptions, emotions, memories and thoughts from ever reaching the conscious mind. This effect has been described as mind expanding, or consciousness expanding, for the drug 'expands' the realm of experience available to conscious awareness. While possessing a unique mechanism of action, cannabis or marijuana has historically been regarded alongside the classic psychedelics. A designer drug is a structural or functional analog of a controlled substance that has been designed to mimic the pharmacological effects of the original drug while at the same time avoid being classified as illegal (by specification as a research chemical) and/or avoid detection in standard drug tests. Many designer drugs and research chemicals are hallucinogenic in nature, such as those in the 2C and 25-NB (NBOMe) families.