Substance

Substance – Heisenberg's suggestion for a common term concerning What cannot Be Destroyed in transitions between matter and energy (though he counsels against assimilating it to the substantia of the ancients)

His central argument: we cannot insist that the common usage of words can be refined to an arbitrary degree of mathematical or scientific acuracy; that they slip aside at the very large scales of relatviity or the very small scales of quantum physics.

We needed it to be a German, and a philosophically inclined one at that, to point this out.