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root; source; origin
foundation; basis; essence
「根本」 has other readings with different meanings.
問題の根本を解決する必要がある。
We need to solve the root of the problem.
根本的な改革なしに変化は望めない。
No change can be expected without fundamental reform.
物事を根本から見直す時期が来た。
The time has come to review things from the ground up.