mainspring

plural mainsprings

1. hobbiesThe principal spring of a clockwork mechanism, which drives it by uncoiling.

   He was wise, for he had seen a long succession of mechanical toys arrive to boast and swagger, and by-and-by break their mainsprings and pass away.

   1921 June, Margery Williams, “The Velveteen Rabbit: Or How Toys Become Real”, in Harper’s Bazar, volume LVI, number 6 (2504 overall), New York, N.Y.: International Magazine Company, →ISSN, →OCLC

2. figurativelyThe most important reason for, or element of, something.

   My daughter is the mainspring of my life.

   2 more examples

   The principles of empiricism and utilitarianism, which lay down that human beings are neither good nor bad by nature but only by upbringing, that interest is the mainspring of morality, and so forth, naturally lead us to inquire what social conditions are necessary to make the community of mankind a reality.

   1978 [1976], Leszek Kołakowski, translated by P[aul] S[teven] Falla, Main Currents of Marxism, volume I: The Founders, Oxford: Clarendon Press, translation of Główne nurty marksizmu: powstanie (in Polish), →ISBN, →OCLC, page 151

   But in the England of Shakespeare’s history plays, the mainspring of the system has broken down.

   2014 November 2, Paul Mason, “What Shakespeare taught me about Marxism”, in The Guardian