If you want to look up a synonym for a certain word, you can go to your thesaurus and check the list of possible alternatives you can use. But that doesn't mean that one should use a thesaurus simply to write fancy, highfalutin words, at least in Martin Amis' view.
In the context of avoiding repetition, he explains that a thesaurus or a dictionary would be useful not to replace a simple word with a more complicated one. Rather, it's all about rhythm.
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