The Introduction to the Science of Sociology is not conceived as a mere collection of materials, however, but as a systematic treatise. On the other hand, the excerpts which make up the body of the book are not to be regarded as mere illustrations. In the context in which they appear, and with the headings which indicate their place in the volume, they should enable the student to formulate for himself the principles involved. An experience of some years, during which this book has been in preparation, has demonstrated the value to the teacher of a body of materials that are interesting in themselves and that appeal to the experience of the student. If students are invited to take an active part in the task of interpretation of the text, if they are encouraged to use the references in order to extend their knowledge of the subject-matter and to check and supplement classroom discussion by their personal observation, their whole attitude becomes active rather than passive. Students gain in this way a sense of dealing at first hand with a subject-matter that is alive and with a science that is in the making. Under these conditions sociology becomes a common enterprise in which all members of the class participate; to which, by their observation and investigation, they can and should make contributions.