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Their confusion is understandable. It’s caused by our reliance on graphs to express quantitative relationships. By plotting y versus x to visualize how one variable affects another, all scientists translate their problems into the common language of mathematics. The rate of change that really concerns them—a viral growth rate, a jet’s velocity, or whatever—then gets converted into something much more abstract but easier to picture: a slope on a graph.
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Steven H. Strogatz (The Joy Of X: A Guided Tour of Math, from One to Infinity)