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1 · Religion and Magic
2 · Nature
3 · Human Being, Man in General
4 · Society, Civilization, Culture
5 · Abstract Ideas and Concepts
6 · History
7 · Bible
8 · Literature
9 · Classical Mythology and Ancient History
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5 · Abstract Ideas and Concepts
51 · Generalities
51G · Some Characteristics of Matter
51G4 · Hardness
51G4(+3) · Hardness (+ symbolical representation of concept)