the reason not just 1, 2, 3 is there are hundreds of course subjects. I actually majored in Psychology, so since you used this subject as an example... 101 and 102 are introductory courses dividing an introductory textbook in half. These courses are for the layperson who doesn't know much about the subject. then 201, 202, 203 is the numbering for the next level. These courses focus on a specific aspect of psychology such as statistics, psychological disorders, cognitive psychology etc. and there are dozens of them. Then these are studied in depth at the upper level of 300 and 400 courses.