§ 41. The rudimentary psychical act, not yet differentiated from a physical act, implies an excitation and a motion. In a creature of low type the touch of food excites prehension. In a somewhat higher creature the odor from nutritive matter sets up motion of the body toward the matter. And where rudimentary vision exists, sudden obscuration of light, implying the passage of something large, causes convulsive muscular movements which mostly carry the body away from the source of danger. In each of these cases we may distinguish four factors. There is (_a_) that property of the external object which primarily affects the organism--the taste, smell, or opacity; and connected with such property there is in the external object that character (_b_) which renders seizure of it, or escape from it, beneficial. Within the organism there is (_c_) the impression or sensation which the property (_a_) produces, serving as stimulus; and there is connected with it, the motor change (_d_) by which seizure or escape is effected. Now Psychology is chiefly concerned with the connection between the relation _a b_, and the relation _c d_, under all those forms which they assume in the course of evolution. Each of the factors, and each of the relations, grows more involved as organization advances. Instead of being single, the identifying attribute _a_, often becomes, in the environment of a superior animal, a cluster of attributes; such as the size, form, colors, motions, displayed by a distant creature that is dangerous. The factor _b_, with which this combination of attributes is associated, becomes the congeries of characters, powers, habits, which constitute it an enemy. Of the subjective factors, _c_ becomes a complicated set of visual sensations co-ordinated with one another and with the ideas and feelings established by experience of such enemies, and constituting the motive to escape; while _d_ becomes the intricate and often prolonged series of runs, leaps, doubles, dives, etc., made in eluding the enemy. In human life we find the same four outer and inner factors, still more multiform and entangled in their compositions and connections. The entire assemblage of physical attributes _a_, presented by an estate that is advertised for sale, passes enumeration; and the assemblage of various utilities, _b_, going along with these attributes, is also beyond brief specification. The perceptions and ideas, likes and dislikes, _c_, set up by the aspect of the estate, and which, compounded and recompounded, eventually form the motive for buying it, make a whole too large and complex for description; and the transactions, legal, pecuniary, and other, gone through in making the purchase and taking possession, are scarcely less numerous and elaborate.