pressed as the enlargement or diminution of the angle of two levers; but chemical actions are not thus expressible; still less vital and mental actions.

13. The organism is on the physical side a mechanism, and so long as the mechanical interpretation of organic phenomena is confined to expressing the mechanical principles involved in the mechanical relations, it is eminently to be applauded. But the organism is something more than a mechanism, even on the physical side; or, since this statement may be misunderstood, let me say, what no one will dispute, that the organism is a mechanism of a very special kind, in many cardinal points unlike all machines. This difference of kind brings with it a difference of causal conditions. In so far as the actions of this mechanism are those of a dependent sequence of material positions, they are actions expressible in mechanical terms; but in so far as these actions are dependent on vital processes, they are not expressible in mechanical terms. Vital facts, especially facts of sensibility, have factors neither discernible in machines nor expressible in mechanical terms. We cannot ignore them, although for analytical purposes we may provisionally set them aside.

In the course of the development of the mechanical theory, the history of which has just been briefly sketched, biological problems have more and more come under its influence. There has always been a fierce resistance to the attempt to explain vital and sentient phenomena on mechanical, or even physical principles, but still the question has incessantly recurred, How far is the organism mechanically interpretable? And while the progress of Biology has shown more and more the machine-like adjustment of the several parts of which the organism is composed, it has also shown more and more the intervention of conditions not mechanically in

terpretable. We shall have to consider the question, therefore, under two forms. First, whether animals are machines, and if not, by what characters do we distinguish them from machines? Secondly, in what sense can we correctly speak of Feeling as an agent in organic processes?

CHAPTER II.

THE VITAL MECHANISM.

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 dif ferent 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 papermill, 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, and therein detected what is common both to its mechanism and to the mechanism of the heavens. Thus equipped, we approach the study of the animal mechanism, but find ourselves wholly in the dark until we have also ascertained its structure and mode of operation; then we may recognize in it the principles of dependent sequence which had been abstracted from the paper-mill and the heavens. To neglect this concrete study, and to argue from Ma

chinery to Life in disregard of special conditions, is not more rational than to assume that the movement of a piston is prompted by volition.

16. The recognition of special differences is no denial of fundamental identities. We do not deny the presence of phenomena in organisms which belong to physical and chemical agencies, but we assert that organisms have other phenomena besides these, dependent on conditions not present in physical and chemical phenomena. The same material elements and forces may be recognized in a moving inorganic body, and a moving organic body; but in the latter there is a speciality of combination with a speciality of result. Just as the same words and laws of grammatical construction may be recognized in prose and poetry; yet poetry is not prose, but has special rules of its own, and special effects. In an organism, as in a machine, the adjustment of the parts is a condition of the mechanical action; the one enables us to explain the other. But the parts adjusted, and the consequences of the adjustment, are unlike in the two cases. This unlikeness is pervading and profound. One cardinal difference is that the combination of the parts is in the machine a fixed, in the organism a fluctuating adjustment; and this fluctuation is due to certain vital processes subjectively known as sensitive guidance. Hence machines have fixed and calculated mechanisms; whereas organisms are variable and to a great extent incalculable mechanisms.

17. I conceive, therefore, that a theory which reduces. vital activities to purely physical processes is self-condemned. Not that we are to admit the agency of any extra-organic principle, such as the hypothesis of Vitalism assumes (Prob. I. § 14); but only the agency of an intra-organic principle, or the abstract symbol of all the co-operant conditions - the special combination of forces which result in organization. This assures us that an