how great intensity of this mood is requisite.  We also hear that poetry is of such a nature that the enjoyment of it is an end in itself; but as it is not maintained that poetry must be wholly impersuasive or uninstructive, there seems to be no means of deciding what amount or prominence of persuasion or instruction would transfer the work to the region of oratory or science.  Such cases make the method of defining by the aid of a type really useful:  the difficulty can hardly be got over without pointing to typical examples of each meaning, and admitting that there may be many divergences and unclassifiable instances on the border between allied meanings.

Sec. 5.  As science began from common knowledge, the terms of the common vocabulary have often been adopted into the sciences, and many are still found there:  such as weight, mass, work, attraction, repulsion, diffusion, reflection, absorption, base, salt, and so forth.  In the more exact sciences, the vague popular associations with such words are hardly an inconvenience:  since those addicted to such studies do not expect to master them without undergoing special discipline; and, having precisely defined the terms, they acquire the habit of thinking with them according to their assigned signification in those investigations to which they are appropriate.  It is in the Social Sciences, especially Economics and Ethics, that the use of popular terminology is at once unavoidable and prejudicial.  For the subject-matters, industry and the conduct of life, are every man’s business; and, accordingly, have always been discussed with a consciousness of their direct practical bearing upon public and private interests, and therefore in the common language, in order that everybody may as far as possible benefit by whatever light can be thrown upon them.  The general practice of Economists and Moralists, however, shows that, in their judgment, the good derived from writing in the common vocabulary outweighs the evil:  though it is sometimes manifest that they themselves have been misled by extra-scientific meanings.  To reduce the evil as much as possible, the following precautions seem reasonable: 

(1) To try to find and adopt the central meaning of the word (say rent or money) in its current or traditionary applications:  so as to lessen in the greater number of cases the jar of conflicting associations.  But if the central popular meaning does not correspond with the scientific conception to be expressed, it may be better to invent a new term.

(2) To define the term with sufficient accuracy to secure its clear and consistent use for scientific purposes.

(3) When a popular term has to be used in a sense that departs from the ordinary one in such a way as to incur the danger of misunderstanding, to qualify it by some adjunct or “interpretation-clause.”