(n.) 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.

ambition aspiration basis calling cause consideration font fount fountain fountainhead goal ground guiding light guiding star headspring headstream headwater ideal inspiration intention lodestar matter mine motive principle quarry reason riverhead sake score source spring springhead ulterior motive vocation well wellhead wellspring