Can an explanation include elements for which there is no evidence of actual existence? An example is the concept of time, used in nearly all scientific theories, but for which no tangible evidence exists. Similar conceptual inventions include Ptolemaic epicycles, genes, (before Watson and Crick) astrology, force fields, phlogistons, string theory and the luminiferous aether. Note that some of these were ultimately defrocked and others eventually became part of the scientific canon. Theories using expedient concepts are called epistemological; theories forbidding such artifice are called realistic.