Material characteristics are built of energy. Substance is energy. Yet no force exists in isolation. What does it mean to be a thing? How can we say what the forces are that constitute the thing itself, since energy disperses completely as reactions produce other forces? What constitutes "thingness"?
Boundaries distinguishing one thing from another seem to be more psychological than physical. We could, for example, view all iron in the universe to be one thing, rather than as a constituent of stars or dust in which iron is found. The classification of things as bodies of closely associated mass is a matter of convenience of perception.
Michael Webb, 2001
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