In human years, psychology as a discipline is just about 18 years old. The discipline is in an early period of maturity, a dawn of adulthood when a few accomplishments look very promising and numerous plans remain daringly ambitious. Like every 18-year-old, psychology once was an infant. Many great thinkers of the past—philosophers and doctors in particular—outlined psychology’s future and helped it to take a few cautious steps forward. Mathematicians, physiologists, and natural scientists guarded psychology during its childhood. Psychology learned the meaning of experiments and measurements. Scientific peers began to acknowledge it. It gained its own voice. First shy and insecure, the discipline of psychology grew stronger with every decade. It began to offer advice and practical solutions to human problems. Accomplishments were noticeable. Setbacks were common. The ambition of many beautiful psychological theories was tamed by the ugliness of stubborn facts. Yet, as in life, these victories as well as mistakes have helped psychology to build its self-confidence. Using the allegory of human years, psychology’s long history is a short but exciting period of growing and maturing. This book is an account of psychology’s maturation over centuries.