For adult man with his mechanized methods of solution, proof is sometimes needed, as here, that an action was a real achievement, not something self-… - Wolfgang Köhler

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For adult man with his mechanized methods of solution, proof is sometimes needed, as here, that an action was a real achievement, not something self-evident; that the breaking off a branch from a whole tree, for instance, is an achievement over and above the simple use of a stick, is shown at once by animals les