With the mysteries of curves and motion now settled, calculus moved on to its third lifelong obsession: the mystery of change. It’s a cliché, but it’s true all the same — nothing is constant but change. It’s rainy one day and sunny the next. The stock market rises and falls. Emboldened by the Newtonian paradigm, the later practitioners of calculus asked: Are there laws of change similar to Newton’s laws of motion? Are there laws for population growth, the spread of epidemics, and the flow of blood in an artery? Can calculus be used to describe how electrical signals propagate along nerves or
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