The principal or most important spring in a piece of mechanism, especially the moving spring of a watch or clock or the spring in a gunlock which impels the hammer. Hence: The chief or most powerful motive; the efficient cause of action.
The imagination is the spur of delights... all depends upon it, it is the mainspring of everything; now, is it not by means of the imagination one knows joy? Is it not of the imagination that the sharpest pleasures arise? Marquis de Sade
Resistance to revelation, resistance to becoming conscious of all that dwells within us - high and low, light and dark - is the anti-change factor. It is also the mainspring of all our psychological fears. Guy Finley
Creatures whose mainspring is curiosity enjoy the accumulating of facts far more than the pausing at times to reflect on those facts. Clarence Day