source of perplexity

See: problem
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References in classic literature ?
I suppose that in all languages the similarities of look and sound between words which have no similarity in meaning are a fruitful source of perplexity to the foreigner.
In their very numbers, Gannett claimed, the indigent poor threaten nothing less than a social contagion: "They are the source of perplexity to the legislator, of trouble to the municipal authorities, and anxious foreboding to the alarmist and the theorist.