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 organism is a peculiar kind of mechanism, the processes in which are peculiar to it; and among those processes there is one which results in what we call Sensibility. This Sensibility is a factor which raises the phenomena into another order. To overlook its presence is fatal to any explanation of the organic mechanism. Yet it is overlooked by those who tell us that when an impression on a nerve is conveyed to the brain, and is thence reflected on the limbs--as when the retina of a wolf is stimulated by the image of a sheep, and the spring of the wolf upon the sheep follows as a “purely mechanical consequence--the whole process has from first to last been physical.” Unless the term _physical_ is here used to designate the _objective sequence_, as contemplated by an onlooker, who likens the process to the sequence observable in a machine, I should say that from first to last the process has been _not_ physical, but _vital_, involving among its essential conditions the peculiarly vital factor named Sensibility. The process taking place in the wolf’s organism is one which involves conditions never found in purely physical processes. We may indeed analytically disregard these. We may view the process in its purely physical relations, or in its purely chemical relations, or in its purely mathematical (mechanical) relations. But this is the artifice of the analytical method. In reality the process is no one of these, for it is all of these; it is a process in a living organism, and depends on conditions only found in living organisms--nay, in this particular case the process depends on conditions only found in organisms like that of the wolf; for the image of the sheep will stimulate the brain of a goat, horse, or elephant without producing any such movement in the organism.