What number is halfway between 1 and 9?

“There’s a whole bunch of different animal species,” says Adam Reeves, a professor of psychology at Northeastern University, “and a whole bunch of different sensory mechanisms, like hearing and vision, and different aspects of all of them, and then taste, and smell, and so on, all of which follow exactly the same law” — a logarithmic relationship between stimulus intensity and perceived intensity. “Biology is very variable, right? So how come all these organisms come up with the same law? And how come the law is so precise? It’s a major philosophical problem, actually.”

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