From these considerations, it is inferred that all systems of mental philosophy will resemble each other just so far as they are true, and that the difference must be mainly in modes of presenting the subject. Inasmuch as writers on mental science are drawing a picture of those experiences of their own minds which are common to the whole race, they must in the main resemble each other, though some may be more imperfect, vague, and disconnected than others.
It may be useful to indicate the causes which have combined to produce perplexity and apparent diversities among writers on mental science.
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.
Meantime the great mass of readers have never been accustomed to any accurate analysis, or even to any fixed observation of their own mental states. They are, therefore, unprepared to detect these defects in the writers on mental science, and are easily confused and perplexed.
Another difficulty has arisen from false ideas as to the origin and proper use of words. In most minds an impression has been generated that there is an inherent meaning belonging to the words of a language. They do not consider that in the formation of language the ideas come first, and that the words are only conventional signs which men agree in using to express these ideas. Writers often speak of words which by long usage have been connected with certain ideas, as if they ought not to be so employed. They do not consider that the fact that men have used a word for a given idea, and understand each other, is the very thing which establishes its proper use and meaning.
If, then, in all time and in all nations, mankind have classified and given names to their mental states, the classification and the names are true and proper, and no philosopher should claim that these are incorrect. The object of language is to enable men to communicate their ideas, and that language is best which enables them to do it the most extensively and the most accurately.