There may sometimes be a difficulty in distinguishing the terms which stand for qualities from those that express activities, since all qualities imply activities:  weight, for example, implies gravitation; and the quality heat is also a kind of motion.  The distinction aimed at lies between a quality as perceived by means of an effect upon our senses (as weight is resistance to our effort in lifting; heat, a sensation when we approach fire), and that property of a body which is conceived to account for its energy (as gravitation that brings a body to the ground, or physical heat that expands an iron bar or works an engine).  The former class of words, expressing qualities, are chiefly used in description:  the latter class, expressing activities, are chiefly needed in explanation.  They correspond respectively, like classification and explanation, with the static and dynamic aspects of Nature.

The terms of ordinary language fall into the same classes as those of science:  they stand for things, classes of things, parts, or qualities, or activities of things; but they are far less precise in their signification.  As long as popular thought is vague its language must be vague; nor is it desirable too strictly to correct the language whilst the thought is incorrigible.  Much of the effect of poetry and eloquence depends upon the elasticity and indirect suggestiveness of common terms.  Even in reasoning upon some subjects, it is a mistake to aim at an unattainable precision.  It is better to be vaguely right than exactly wrong.  In the criticism of manners, of fine art, or of literature, in politics, religion and moral philosophy, what we are anxious to say is often far from clear to ourselves; and it is better to indicate our meaning approximately, or as we feel about it, than to convey a false meaning, or to lose the warmth and colour that are the life of such reflections.  It is hard to decide whether more harm has been done by sophists who take a base advantage of the vagueness of common terms, or by honest paralogists (if I may use the word) who begin by deceiving themselves with a plausible definiteness of expression, and go on to propagate their delusions amongst followers eager for systematic insight but ignorant of the limits of its possibility.

Sec. 3.  A Definition is necessary (if possible) for every scientific name.  To define a name is to give a precise statement of its meaning or connotation.  The name to be defined is the subject of a proposition, whose predicate is a list of the fundamental qualities common to the things or processes which the subject denotes, and on account of possessing which qualities this name is given to them.