The word crucial entered English in the 18th century in the sense ‘cross-shaped’, coming via French from Latin crux (stem cruc-) ‘cross’. The sense ‘decisive’ developed from the Latin phrase instantia crucis ‘crucial instance’, coined by the philosopher Francis Bacon (1561–1626): Bacon was using the crux, or fingerpost marking a fork at a crossroads, as a metaphor for the moment at which one has to take a decision. The scientists Sir Isaac Newton and Robert Boyle took up the metaphor in experimentum crucis ‘crucial experiment’. Latin crux is the root of a number of words, including cross, crucible, crucify, cruise, crusade, and excruciate.