"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 preëminently the nervous system, while contractility, or the vis musculosa, is the name of the special property of the muscular tissue.
"Considering them however in their origin, they together represent a certain form of the transmission of force.... Some kinds of impressed force are followed by movements of retraction and withdrawal, others by such as secure a continuance of the impression. These two kinds of contraction are the phenomena and external marks of pain and pleasure respectively. Hence the tissue acts so as to secure pleasure and avoid pain by a law as truly physical and natural as that whereby a needle turns to the pole, or a tree to the light.... Hence, the law of Self-Conservation, or of the direction of Action, is merely another mode of expressing the fundamental property of animal tissue, which we have every reason to believe is derived from the more elementary physical properties of matter. The course of action is just as dependent on physical laws as that of a stone which falls to the ground. The belief in external consciousness makes no difference either way; the earliest phenomena of such consciousness are those of pleasure and pain, therefore we can suppose it to exist only as pleasure and pain. In the one case we say that action aims at, or naturally results in, the phenomena of pleasure; in the other case that it aims at the actual consciousness of pleasure.
"The expression of impressed force, or the connection of action and sensation, is at first in the unorganized tissue direct and immediate, without the agency of nervous communication, or to return again to the ordinary psychological language, is unintentional or involuntary.... The earliest modification is due to association, whereby secondary sensations, or (as they are called later when they become perceived) ideas are produced. These manifest themselves as weaker repetitions of the primary pleasures and pains, and, therefore, are naturally followed by like results.... The process is this: the force originally impressed by the first sensation, instead of being all expressed in action, is partly induced by habituation into an internal channel, and so transformed into the kind of force which generally impresses the second kind of sensation, and this now produces its appropriate action. Hence part of the original force has undergone two transformations instead of one; the immediate antecedent of action being the force produced by association, or in other words, the associated pleasure. This is the rudiment of motive, which, however, is not generally called by that name till it is perceived. The same process may go on through two or more links of association; the first transformed force being again transformed internally instead of expressed, and the second again in its turn, until eventually a transformation is reached which finds its easiest way of escape in action; the immediate motive power being that transformation of force, or that associated pleasure, which immediately precedes the action. Actions of this kind constitute the lower phenomena of instinct: and we see therefore that they may depend on any number of links of unperceived, or, as we say, unconscious reasoning; and that their motive is also 'unconscious.' These actions stand half way between Reflex and Voluntary Actions....
"We now come to the third and last development of associated action. Here not only is each associated idea perceived, but the change, in each case, is also a fresh centre of association; whereby similar changes are connected with it, and it is referred to a class. Hence the whole train is perceived, not only by the classification of each of its parts with similar previous sensations, but by the classification of each of its sequences with previous like sequences: in other words, it is now a chain of reasoning from the past to the present. That associated pleasure from which this reasoned train commences is now called the motive (though really the immediate motive power lies in the last transformation which directly precedes the active expression) and the series of ideas intervening between this and the action is called the means. Hence the motive associates the means, and the motive power is transmitted through them till it is finally expressed in the action which is appropriate to the attainment of the pleasurable state whose idea is its source. This association of means with ends is at first sight opposed to the natural direction, which is from antecedent to consequent; but when a line of nervous connection is formed, a current may be transmitted indifferently in either direction. An effect may lead us to think of its cause, as easily as a cause associates its effect. By the sequence of action and sensation, a connection is established between their ideas, which is independent of the order of excitation. This last kind of action is that which we call voluntary, and the series of classified ideas and relations which lead to it is called Reasoning. If at any point the current is attracted in two or more directions by different trains of association, deliberation is the result; and the eventual victory of one and the consequent transmission of the force along it is entitled Will.
"We have therefore distinguished four kinds of action: Reflex Action, which is purely physical and independent of association, and which is the last link in all the derived varieties; Lower Instinctive Action, which is caused by the first introduction of association, and is hardly to be distinguished in its phenomena from the last;... Higher Instinctive Action, which involves perception of qualities or objects;... and finally, Voluntary or Intentional Action, such as we find it in man.... Though we have separated these classes from each other for clearness of description, there is no distinct line to be drawn anywhere between them. Each fades insensibly into the next.... Evolution, we must remember, does not advance by stages; these are merely marks that we make ourselves, like the constellations in astronomy, for convenience of study.
"Finally, we must remark that the last two kinds of Action ever tend to relapse into the second, which subjectively is a mere form of the first. Association of all kinds tends to become organic. By this we mean that, as the connection becomes more definitely marked and easy, the perpetual radiation which occurs as the current passes the different points on its path, disappears; and the whole current passes unimpaired. First, the radiation caused by the changes disappears, and reasoning becomes instinct, as in doing a mathematical example from mere memory of the different steps. Secondly, the radiation from the different nervous centres also disappears, and the current which ends in action becomes not only unreasoning but unperceived, as in walking or reading aloud while thinking of something else....
"Long habituation has two effects: it increases the number of trains connected with each object, and also the length of each. If we suppose the simpler emotions to have, by this time, become organic or apparently simple states of consciousness, a continuance of association tends to connect them together in bundles, as they themselves were originally bundles of elementary pleasures and pains. Hence the emotions become organized in their turn so as to form higher emotions, and eventually, when association has completed its work,... this organization ends in one supreme emotion, which is the head of the emotional or sensitive side of the consciousness....
"Turning next to the second effect of prolonged habituation, we find that, with objects or actions with which pleasure was at first associated and which so were called pleasurable, further association often connects a subsequent pain which increased experience has shown always to follow upon the immediate pleasure. This pain often more than counterbalances the preceding pleasure; hence when it is taken into the emotion, that emotion becomes one no longer of appetition but of aversion, and the object or action is remembered as one not to be sought after but avoided. It cannot, however, be called painful, because it causes immediate pleasure, so a new name has to be invented, and it is called Bad, or Evil. Similarly, many things which are immediately associated with pain are found to be eventually followed by pleasure which more than counterbalances the pain, and as this experience becomes consolidated by the power of association, they attract rather than repel, and for a name whereby to distinguish them, are called Good; so that Good and Evil are correlative terms like Pleasure and Pain, and mean respectively the greatest total Pleasure, and the greatest total Pain. Now this experience when once acquired is never lost, but by virtue of hereditary transmission descends from parents to children. But, as in the case of the simpler emotions, only the results survive, and not the means whereby they were arrived at; so that, in a short time, the words Good and Evil come to be quite separated from Pleasant and Painful; nay, as might be expected from their origin, they tend to acquire exactly opposite meanings; for Pleasure and Pain come to signify only immediate pleasure and pain; and the final reckoning is often considerably at variance with the first item; as in a race the man who leads for the first lap seldom wins in the end....
"This, then, is the origin of the Moral Sense.... The Moral Sense, therefore, is merely one of the emotions," though the last of all in the order of evolution; it can only claim a life of some two or three centuries; and there are even some who still doubt its existence. "Man at any rate is the only animal who possesses it in its latest development; for even in horses and dogs we cannot believe that it has passed the intentional or conscious stage.... Good, with them, has no artificial meaning; it is simply identical with the greatest pleasure."
Only by complete and perfect obedience to all emotions can perfect freedom from regret be obtained in the gratification of all desire. Man is at present passion's slave, because he is so only in part; "for the cause of repentance is never the attainment of some pleasure, but always the non-attainment of more: not the satisfaction of one desire, but the inability to satisfy all. The highest virtue, therefore, consists in being led, not by one desire, but by all; in the complete organization of the Moral Nature."