An idea becomes a scientific idea when it functions in the context of a scientific explanation. The idea of time is an excellent example. If, as Plato claimed, both real knowledge and ultimate reality are timeless, then time is insignificant. However, by the 18th century, the idea of time was increasingly regarded as the dimension containing hope for an improvement in the human condition. This, in turn, prefigured 19th-century scientific ideas of time as both irreversible and significant.