Still earlier in man's development, before he could write, it was customary to record such agreements by breaking a stick in two pieces and leaving the jagged ends to be fitted together at time of fulfillment. Sometimes a bone was used this way. Because its critical feature was the saw-toothed edge, this kind of contract was called indenture (derived from the root _dent_--tooth, the same one from which we derive our word dentist). The formal, legal-looking document which we today call an indenture gives us no hint of its humble origin, but the word when analyzed by the technique of philology tells the whole story, and throws much light upon the legal practices of our forbears. Having discovered one such valuable fact in philology, the student of law may be led to investigate the science still further and find many more. As a result still he will become interested in philology. By this illustration we have demonstrated the first psychological law of interest, and also its corollary which is: _State the new in terms of the old_. For we not only gave our lawyer new information culled from philological sources; we also introduced our fact in terms of an old fact which was already "interesting" to the lawyer. This is recognized as such an important principle in education that it has become embodied in a maxim: Proceed from the known to the unknown. A classic example of good educational practice in this connection is the way in which Francis W. Parker, a progressive educator of a former generation, taught geography. When he desired to show how water running over hard rocky soil produced a Niagara, he took his class down to the creek behind the school house, built a dam and allowed the water to flow over it. When he wished to show how water flowing over soft ground resulted in a deltoid Nile, he took the class to a low, flat portion of the creek bed and pointed out the effect. The creek bed constituted an old familiar element in the children's experience. Niagara and the Nile described in terms of it were intelligible. Naturally in modern educational practice it is not always possible to have miniature waterfalls and river bottoms at hand, still it is possible to follow this principle. When, in studying Mediaeval History, you read a description of the guilds, do not regard them as distant, cold, inert institutions devoid of significance in your life. Rather, think of them in terms of things you already know: modern Labor Unions, technical schools, in so far as the comparison holds good. Then trace their industrial descendants down to the present time. By thus thinking about the guilds, hitherto distant and uninteresting, you will begin to see them suffused with meaning, alight with significance, a real part of yourself. In short, you will have achieved interest.