A good example of a vague concept is the concept of a heap of sand -- two or three grains of sand is not a heap, but a thousand is. So how many grains of sand does it take to make a heap? There is no clear line. It appears to be paradoxical to ask at what point a heap stops being a heap as grains are removed. This is known as the paradox of the heap or the sorites paradox (from the Greek language word for "heap").
... - first to navigate down the Amazon River
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William Parry[?], (born 1790), Arctic explorer
Robert Edwin Peary, (1856-1920), first man to reach the North Pole
Auguste ...