The three chief principles stated--The first principle--Serviceable actions become habitual in association with certain states of the mind,and are performed whether or not of service in each particular case--The force of habit--Inheritance--Associated habitual movements in man--Reflex actions--Passage of habits into reflex actions--Associated habitual movements in the lower animals--Concluding remarks.
　　I WILL begin by giving the three Principles,which appear to me to account for most of the expressions and gestures involuntarily used by man and the lower animals,under the influence of various emotions and sensations. I arrived,however,at these three Principles only at the close of my observations.They will be discussed in the present and two following chapters in a general manner.Facts observed both with man and the lower animals will here be made use of;but the latter facts are preferable,as less likely to deceive us.In the fourth and fifth chapters,I will describe the special expressions of some of the lower animals;and in the succeeding chapters those of man.Everyone will thus be able to judge for himself,how far my three principles throw light on the theory of the subject.It appears to me that so many expressions are thus explained in a fairly satisfactory manner,that probably all will hereafter be found to come under the same or closely analogous heads.