An early effort to shape complex behavior was suggested by an experiment in which a chimpanzee used poker chips to operate a sort of vending machine.1 It was implied by some writers that in doing so the chimpanzee demonstrated certain higher mental processes—in which, for example, the poker chip served a symbolic function. To show that operant conditioning could explain the behavior, I conditioned a rat to behave in a similar way. (p. 323)
