What is a “concrete” word? It is a spell-symbol used to tag something you can point to and thereby agree on, e. g., “cat,” “sailboat,” “ice-skating”—… - Robert A. Heinlein

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What is a “concrete” word? It is a spell-symbol used to tag something you can point to and thereby agree on, e. g., “cat,” “sailboat,” “ice-skating”—agree with such certainty that when you say “sailboat’’ there is no chance whatever that I will think you mean a furry quadruped with retractile claws.
With the spell symbol “God” there is no way to achieve such agreement because there is nothing to point to.