Series creator
Rod Serling made up the phrase "Sixth Dimension" to use in season one's opening narration. William Self of CBS-TV asked him what was the fifth dimension (given that dimensions one through three are exemplified by a line, a plane, and a cube, respectively, and the fourth is time). Serling answered, "I don't know. Aren't there five?" He then changed the narration to "There is a fifth dimension..."
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