Cornerstone and Mainspring

Cornerstone noun - An immaterial thing upon which something else rests.

Mainspring and cornerstone are semantically related in foundation topic. In some cases you can use "Mainspring" instead a noun "Cornerstone", when it comes to topics like vital element.

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

Cornerstone and mainspring are semantically related in essence topic. You can use "Cornerstone" instead a noun "Mainspring", if it concerns topics such as theme.