The only way to discover the nature of a thing is to examine what are its qualities, how it acts, and how it is acted upon. This also is the same as studying the _philosophy_ of things. For when learned men set forth any branch of philosophy, they only teach the qualities of certain things, how they act, and how they are acted upon. Whoever, therefore, gives attention to the nature of mind so as to discover its qualities, how it acts, and how it is acted upon, is studying the philosophy of mind, or mental philosophy. The _nature of mind_, _the philosophy of mind_, and _mental philosophy_ are terms all expressing the same thing. Now, the only possible way in which any person can discover the nature of another mind is by a knowledge of his own. We first learn by experience the qualities of our own mind, how it acts and how it is acted upon, and then, by a process of reasoning, we learn that there are other minds around us, and that they have similar qualities. The study of mental philosophy, then, is directing attention to the nature of our own mind, and thus discovering the nature of other minds. It differs from all other studies in this respect, that all men have the materials of the knowledge sought in their own minds, and are required simply to direct attention to their own mental states and acts. This being so, the common people are as fully qualified to settle all questions in regard to the nature or philosophy of their own minds as the most learned and profound metaphysicians or theologians can be. All that is requisite to success is, that they direct their attention to the subject by suitable methods. It will be found, on examination, that the common people have secured a written system of mental philosophy as real as has ever yet been furnished by any metaphysician or theologian, while it is free from the great defects which render many works on mental science unpractical and repulsive. This—_the people’s system of mental philosophy_—it will be the object of what follows to set forth. In attempting it, we shall find that mankind, in the uses of every‐day life, have arranged the various acts and states of mind into classes and subdivisions, and have given names to these classes, and to the specific acts or states included in these classes. These classifications and terms are recorded by lexicographers in their dictionaries. All words have that meaning which is attached to them by the people who use them. The business of the lexicographer is, not to settle what meaning ought to belong to words, but rather to state the meaning which men actually attach to them in writing and speaking.