Originally published in 1948, the author follows the idea that the instincts are the spring and basis of all animal behaviour (with the exception perhaps of play), and therewith the core of the animal's mind, and that individual experience, gathered by the animal in the course of its life, may influence and reconstruct these instincts, so as to guide, in the form of intelligence and understanding, this behaviour along new (i.e. innate) paths. Thus, instinct and experience become the pillars upon which animal behaviour is built up; instinct, intelligence, and understanding form a triad round which the facts of the psychology of animals may be grouped. As a foundation of all this the author first tries to prove the good right of a real and genuine animal psychology, not hampered by objectivistic and behaviouristic scruples, while in a final chapter, by way of conclusion, he tries to give an image of how the world of the animal is built up.