Why did James Prescott Joule’s theory of heat and motion challenge accepted scientific ideas?

James Prescott Joule’s theory challenged the prevailing scientific consensus of the mid-19th century, which was rooted in the caloric theory.

At the time, heat was widely accepted as an "imponderable substance" or an imaginary, weightless fluid called caloric that flowed between bodies. Joule’s work contradicted this by proposing that heat was not a substance, but rather a form of energy related to mechanical motion.123

Joule demonstrated that heat could be generated by doing work on a substance and that various forms of energy—mechanical, electrical, and heat—were interconvertible. This discovery provided the foundation for the first law of thermodynamics, which states that energy cannot be created or destroyed, only transformed. While initially met with skepticism by contemporaries like William Thomson, Joule’s theory eventually replaced the caloric model, fundamentally changing the scientific understanding of energy transfer and physical processes.3142