mainspring (n.)

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