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Silent Spring, Rachel Carson’s landmark warning about the indiscriminate use of pesticides, turns 50 this month. By extension, that puts the environmental movement also at the half-century mark—along with the bitter, divisive argument we continue to have over both the book and the movement it spawned. The terms of that argument, which emerged in the brutal reaction to Silent Spring from those who saw it not as a warning but as a threat, haven’t changed much. And they leave us with a vexing question: Why do we fight?