Full Idea
Helmholtz crucially asserted that all forces conserve the sum of kinetic and potential energy; superficially non-conservative forces like friction are simply macroscopic manifestations of more fundamental forces conserving energy at the micro-level.
Gist of Idea
All forces conserve the sum of kinetic and potential energy
Source
report of Hermann von Helmholtz (On the Conservation of Force [1847]) by David Papineau - Thinking about Consciousness App 4.3
Book Reference
Papineau,David: 'Thinking about Consciousness' [OUP 2004], p.246
A Reaction
Friction had been a problem case, because it appeared not to conserve energy when it slowed movement down.