Full Idea
The first law of thermodynamics : energy can be changed from one form to another, but is always conserved overall.
Gist of Idea
First Law: energy can change form, but is conserved overall
Source
Frank Close (Theories of Everything [2017], 3 'Perpetual')
Book Reference
Close,Frank: 'Theories of Everything' [Profile Books 2017], p.27
A Reaction
So we have no idea what energy is, but we know it's conserved. (Daniel Bernoulli showed the greater the mean energy, the higher the temperature. James Joule showed the quantitative equivalence of heat and work p.26-7)