Mainspring and Source

Mainspring noun - The most important spring in a mechanical device (especially a clock or watch); as it uncoils it drives the mechanism.

Source is a synonym for mainspring in cause topic. In some cases you can use "Source" instead a noun "Mainspring", when it comes to topics like essence, origin, agent, base.

Source noun - A point or place at which something is invented or provided.

Mainspring is a synonym for source. You can use "Mainspring" instead a noun "Source".