Full Idea
The mass of a body is the measure of its energy content.
Gist of Idea
Mass is a measure of energy content
Source
Albert Einstein (works [1915]), quoted by Peter Watson - Convergence 04 'Intro'
Book Reference
Watson,Peter: 'Convergence' [Simon and Schuster 2016], p.108
A Reaction
If I knew what energy was, this would be very illuminating. This idea is e=mc^2 in words. We now have the Higgs field to consider when trying to understand mass.