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 interpretable. 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.