Feynman's analogy of of energy conservation to accounting for indestructible
blocks is at
Feynman Lectures on Physics, Vol 1, 4-1 through 4-2.
Interestingly, following Feynman's "there are no blocks" a few lines later
he states: "...when we are calculating the energy, sometime some of it
leaves the system and goes away, or sometime some comes in. In order to
verify the conservation of energy, we must be careful that we have not put
any in or taken any out."