The first cause is the want of an accurate medium of communication by which one mind can compare its experience with the experience of other minds. In natural science, when the philosopher instructs in reference to the properties of matter, all the terms employed can be made definite by appeals to the senses. For example, if it is not understood what is meant by a _pungent_ smell, such a smell can be produced, and then there is a perfectly clear idea of what is meant by the term. But in mental science, when the term _reason_ or the term _understanding_ is employed, no such perfect and definite mode is at command to illustrate the meaning. On the contrary, in this science, a single term is often used with various meanings, each use, however, including some common idea, while the extent or limitation in every case is to be determined by the connection. For example, the term _heart_ is used sometimes to signify the chief organ of physical life, sometimes it signifies the mind itself. In a more limited use it denotes the feelings, and in a still more restricted sense it expresses the leading interest of the mind. This involves a constant process of reasoning to decide the meaning of the term. Another perplexity in mental science has arisen from an unwarrantable use of terms by writers. In some instances new distinctions in mental analysis have been originated, and then terms have been used to express these distinctions which never before were employed in this limited sense. Of course, in reading their works, the mind is confused by meeting terms that in common use recall one signification, when the writer employs them in another. In other cases, such writers have formed new classifications of mental phenomena, and employed new terms to express them, and thus an impression is made that something new has been discovered, or a new system evolved. For example, Brown arranges the intellectual operations of mind in but two general classes, and calls them _simple suggestion_ and _relative suggestion_. But his work, in this respect, presents only a new classification and new terms, but no new ideas. Another difficulty in mental science has arisen from the fact that many writers on this subject have failed in accurate analysis of the phenomena of mind, and, of course, have not succeeded in conveying clear and distinct ideas to their readers. For example, some metaphysicians have never discriminated between _desire_ and _choice_, but have written as if they were the same thing. Thus they have affirmed things which were true in reference to one of these mental acts, and false in regard to the other. This has produced mistiness of apprehension or false conceptions in their readers. Some understand the writer one way and dispute his positions, others understand him another way and defend them, because what he says is true of one act and false of the other, while both acts are spoken of as one and the same.