At nearly the same time, a slender volume by Valentino Braitenberg, called
Vehicles: Experiments in Synthetic Psychology,25 appeared. It was a series of
gedanken or entirely hypothetical thought experiments, speculating as to
how machine intelligence could evolve. Braitenberg started with simple
thermal sensor-motor actuator pair (Vehicle 1) that could move faster in warm
areas and slower in cold areas. The next,more complex vehicle had two thermal
sensor-motor pairs, one on each side of the vehicle. As a result of the
differential drive effect, Vehicle 2 could turn around to go back to cold areas.
Throughout the book, each vehicle added more complexity. This layering
was intuitive and seemed to mimic the principles of evolution in primates.
Vehicles became a cult tract among roboticists, especially in Europe.