Aristotle’s first period in Athens had lasted for twenty years. In 347 he suddenly left, probably for political reasons. During his travels, 347–35, he undertook most of the work on which his scientific reputation rests. He made or collected observations in astronomy, meteorology, chemistry, physics, psychology, and other sciences; but his scientific fame rests primarily on his work in zoology and biology. According to ‘Zoological researches’, his studies on animals laid the foundations of the biological sciences and were not superseded for over two millennia. His History of Animals is not flawless, but it is a masterpiece. Nowhere else does Aristotle show more vividly his ‘desire to know’.