14. No answer can be successfully attempted in reply to the first of the questions which closed the last chapter until we have given precision to certain terms of incessant recurrence. I have often to remark on the peculiar misfortune of Psychology, that all its principal terms are employed by different writers, and are understood by different readers, in widely different senses: they denote and connote meanings of various significance. All physicists mean the same thing when they speak of weight, mass, momentum, electricity, heat, etc. All chemists mean the same thing when they speak of affinity, decomposition, oxygen, carbonic acid, etc. All physiologists mean the same thing when they speak of muscle, nerve, nutrition, secretion, etc. But scarcely any two psychologists mean precisely the same thing when they speak of sensation, feeling, thought, volition, consciousness, etc.; and the differences of denotation and connotation in their uses of such terms lead to endless misunderstanding. As Rousseau says: “Les définitions pourraient être bonnes si l’on n’employait pas les mots pour les faire.” But since we must employ words as our signs, our utmost care should be given to clearly marking what it is the signs signify. 15. The question we have now before us, whether animal actions are interpretable on purely mechanical principles? can only be answered after a preliminary settlement of the terms. The first of these terms to be settled is that of mechanism, when applied to the vital organism. If the organism is a mechanism, its actions must of course be interpretable on mechanical principles. But this general truth requires a special interpretation, if on inquiry we find that the organism is a particular kind of mechanism, one which is _not_ to be classed under the same head as inorganic machines. And this we do find. In Problem I. § 22, will be found a statement of the radical difference between organic and inorganic mechanisms, due to the differences in their structures. But the differences there noted do not affect the operation of abstract mechanical principles, which are of course manifested _wherever_ there is a dependent sequence of material changes; and which are the same abstract principles in the mechanism of the heavens, the mechanism of a paper-mill, or the mechanism of an animal body. In other words, the principles are abstract, and are abstracted from all concrete cases by letting drop what is special to each case, retaining only what is common to all. This procedure is indispensable to the ideal constructions of Science. But we cannot rightly interpret any concrete case by abstract principles alone; we must restore the special characters which the abstraction has eliminated. The most lucid explanation of the mechanism of the heavens will leave us quite in the dark respecting the action of a paper-mill, until we have studied the mill at work, ascertained its structure and mode of operation,