The philosopher king is a hypothetical ruler in whom political skill is combined with philosophical knowledge. The concept of a city-state ruled by philosophers is first explored in Plato's Republic, written around 375 BC. Plato argued that the ideal state – one which ensured the maximum possible happiness for all its citizens – could only be brought into being by a ruler possessed of absolute knowledge, obtained through philosophical study. From the Middle Ages onwards, Islamic and Jewish authors expanded on the theory, adapting it to suit their own conceptions of the perfect ruler.