driving force
The impetus, power, or energy behind something in motion, as in He was clearly the driving force in the new administration. This term transfers the force that sets in motion an engine or vehicle to other enterprises. Ralph Waldo Emerson was among the first to use it figuratively (English Traits, 1856): “The ability of its journals is the driving force.”
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
Examples from the Web for driving force
Historical Examples
Learning is only possible when instinct supplies the driving-force.
The Analysis of MindBertrand Russell