During the war, I worked in aircraft design. About a year after D-day, an exhibition was arranged at Farnborough of the mass of German equipment that had been captured, including the doodlebug and the V2 rocket. I and a friend spent a fascinating two days wandering round the exhibits. The questions that kept arising were ‘Why did they make it like that?’, or, equivalently ‘I wonder what that is for?’ We were particularly puzzled by a gyroscope in the control system of the V2. One's first assumption was that the gyroscope maintained the rocket on its course, but, instead of being connected to the steering vanes, it was connected to the fuel supply to the rocket. Ultimately my friend (who would have made a better biologist than I) remembered that the rate of precession of a gyro depends on acceleration, and saw that the Germans had used this fact to design an ingenious device for switching off the engines when the required velocity had been reached.

For an engineer, this was thinking backwards. Typically, an engineer wants to produce a result, and devises machinery to do it; on this occasion we were looking at machinery, and trying to deduce what result it was designed to produce. Many years later, I came to appreciate that biologists spend their lives thinking backwards in this way. We see a complicated structure—a wing joint, a ribosome, the cerebellum—and ask not only how it works, but what it is for. This sharply distinguishes biology from physics and chemistry. Of any particular phenomenon, we ask two questions, whereas they ask only one.