"Now by various trains of association, various pleasures and pains are connected with the same object. These different combinations of pleasures and pains, some of which arise, before reasoning, by unintentional association, but the higher of which are the results of automatization of reasoning, form the different emotions.... "Action in its origin is simply the correlative of sensation. Contractility and irritability are the two general properties of vital tissue, or rather are two sides of one fundamental property which is also known under the name of sensibility--the power of contraction under irritation, or of expressing impressed force. Irritability means merely the phenomena of consciousness, the development of which we have hitherto been tracing, though we have been throughout obliged to express ourselves in the language of the inner, and not of the outer experience.... This internal development we have already examined; we must now turn to the obverse external development which takes its origin in contractility. "The connection between these two fundamental properties is exceedingly intimate, that of ultimate identity or at any rate inseparability. For not only is contraction universally the result of irritation, but the only evidence that we have of irritation is the contraction which follows, and in their early stages the two represent one and the same process. When, however, the expression, in action, of force impressed in sensation, becomes indirect and immediate, the name of irritability is given to the _immediate_, internal results of its impression, while contractility expresses the action _ultimately_ expressed. Hence the seat of irritability is preeminently the nervous system, while contractility, or the _vis musculosa_, is the name of the special property of the muscular tissue.