Link R. Swanson divides scientific frameworks for understanding psychedelic experiences into two waves. In the first wave he includes model psychosis theory (the psychotomimetic paradigm), filtration theory, and psychoanalytic theory. Aldous Huxley was a proponent of filtration theory. In his book "The Doors of Perception", he presents the idea of a mental reducing valve in order to explain the significance of the psychedelic experience. According to Huxley, the central nervous system's main function is to shut out the majority of what we perceive; the brain filters those perceptions which are useful for survival. Society aids in this filtering by creating a symbolic system which structures our reality and which reduces our awareness. Huxley postulated that psychedelics lessened the strength of the mind's reducing valve, allowing for a broader spectrum of one's overall experience to enter into conscious experience.