If you look at encyclopaedias back in the 1800s, you will see the definition of psychology, as it is the most proper introduction to speculative philosophy. Since then, internal (conceptual, methodological) and external (social, political) factors have shaped the development of psychology as a scientific discipline. Today what we understand psychology is the scientific study of thought, emotions, and behaviours. Voyage of mysteries of human thought, emotions, and behaviours through scientific methods in this course provides us exciting discoveries of our potential, limits, successes, failures, and true nature of a human being. It also gives us the knowledge to achieve better services and peaceful societies and to improve every aspect of individuals' lives in the modern world. It is essential to have the ability and awareness for differentiating "common sense" and research evidence in the 21st-century complex modern world. Based on scientific research, understanding the reasons for some fundamental questions such as why we think in a particular way, why we do, and what we do can be extremely helpful in your business life as well as your academic achievement. Therefore, knowing essential debates in the psychological foundation would be necessary for every psychologist who would like to enhance her or himself in the contemporary global world.