It's a simple enough question, it seems: Is an apple living? First, define “life.” Then apply the definition. Finally, arrive at a definitive answer. Isn't that clarity one of the great virtues of scientific knowledge?

But seasoned biologists know well that “life” is not easily defined. Like other scientific concepts, a single, apparently simple term hides substantial complexity. Others may realize that too, with just a little bit of reflection. For example, imagine using the question about apples to open the first day of a biology class and to spark a lively independent discussion among students:

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What is life, indeed? What seems obvious at first, may not be....

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