From Longman Dictionary of Contemporary Englishthe mainspring of somethingthe mainspring of somethingEFFECT/INFLUENCEthe most important reason or influence that makes something happen Christian faith was the mainspring of Peter’s life. Small companies are the mainspring of the British economy. → mainspring
Examples from the Corpus
the mainspring of something• This double vision of the woman-goddess is said to be the mainspring of Shakespeare's tragedies.• The villagers' fierce independence formed the mainspring of revolt.• It so happened that the mainspring of my clockwork mouse had broken that very morning.• Kitson's great organizing ability, technical ingenuity, and grasp of industrial developments was the mainspring of these activities.• Also there was the mainspring of the enterprise, the catering team. 