In the early stages of observing chimps, it was clear that the examinations should not be considered characteristic for each member of this species. Köhler recognized that, as in humans, chimpanzees demonstrated considerable differences in the intellectual field. Chimps demonstrated that they were able to grasp the objects around them in a variety of fashions. This is incorporated in their everyday playing behaviours. For this reason, it was not necessary to use experimental tests to introduce chimps to handle matter. In his book, "The Mentality of Apes", Köhler describes how the apes use their hands, saying "large, powerful and flexible hands are natural links between himself and the world of things, and he attains the necessary amount of muscular force and co-ordination at an earlier age than the human child".