In 1913, Köhler left Frankfurt for the island of Tenerife in the Canary Islands, where he had been named the director of the Prussian Academy of Sciences anthropoid research station. He worked there for six years, during which he wrote a book on problem solving titled "The Mentality of Apes" (1917). In this research, Köhler observed how chimpanzees solve problems, such as that of retrieving bananas when positioned out of reach. He found that they stacked wooden crates to use as makeshift ladders, in order to retrieve the food. If the bananas were placed on the ground outside of the cage, they used sticks to lengthen the reach of their arms. Köhler concluded that the chimps had not arrived at these methods through trial-and-error (which American psychologist Edward Thorndike had claimed to be the basis of all animal learning, through his law of effect), but rather that they had an insight, in which, having realized the answer, they proceeded to carry it out in a way that was, in Köhler's words, "unwaveringly purposeful."