mainspring (man spring)

1. The principal spring of any piece of mechanism, as, in a gun-lock, the spring which operates the hammer; specifically, the coiled spring of a watch or other timepiece. God 's the mainspring , that maketh every way All the small wheels of this great Engine play. Sylvester, tr. of Du Bartas's Weeks, i. 7. Hence-2. The impelling cause of any action; the inciting motive. It was no longer the savage love of plunder or the nece ities of providing subsistence, the mainspring of the barbarian's inroads, that excited men to war-like enterprise. Brougham.