"According to the idea generally attached to the word _instinct_ the faculty which this word expresses is considered as a light which illuminates and guides animals in their actions, and which is with them what reason is to us. No one has shown that instinct can be a force which calls into action; that this force acts effectively without any participation of the will, and that it is constantly directed by acquired inclinations." There are, the author states, two kinds of causes which can arouse the inner feeling (organic sense)--namely, those which depend on intellectual acts, and those which, without arising from it, immediately excite it and force it to direct its power of acting in the direction of acquired inclinations. "These are the only causes of this last kind, which constitute all the acts of _instinct_; and as these acts are not the result of deliberation, of choice, of judgment, the actions which arise from them always satisfy, surely and without error, the wants felt and the propensities arising from habits. "Hence, _instinct_ in animals is an inclination which necessitates that from sensations provoked while giving rise to wants the animal is impelled to act without the participation of any thought or any act of the will. "This propensity owes to the organization what the habits have modified in its favor, and it is excited by impressions and wants which arouse the organic sense of the individual and put it in the way of sending the nervous fluid in the direction which the propensity in activity needs to the muscles to be placed in action. "I have already said that the habit of exercising such an organ, or such a part of the body, to satisfy the needs which often spring up, should give to the subtile fluid which changes its place where is to be operated the power which causes action so great a facility in moving towards this organ, where it has been so often employed, that this habit should in a way become inherent in the nature of the individual, which is unable to change it. "Moreover, the wants of animals possessing a nervous system being, in each case, dependent on the Structure of these organisms, are: "1. Of obtaining any kind of food; "2. Of yielding to sexual fecundation which excites in them certain sensations; "3. Of avoiding pain; "4. Of seeking pleasure or happiness. "To satisfy these wants they contract different kinds of habits, which are transformed into so many propensities, which they can neither resist nor change. From this originate their habitual actions, and their special propensities to which we give the name of instinct.[187]