Lastly, thought may be the source of animal motion, and yet there may be no need of a distinct thought producing each individual motion. This is a very essential point in the subject before us. In uttering a cry for example, the number of muscles and articulations of the body concerned in this operation is very great; shall we say that the infant has a distinct thought for each of these articulations?
The answer to this question will be considerably facilitated if we recollect the manner in which the impressions are blended which we receive from external objects. The sense of feeling is diffused over every part of my body, IChapter eel the different substances that support me, the pen I guide, various affections and petty irregularities in different parts of my frame, nay, the very air that environs me. But all these impressions are absolutely simultaneous, and I can have only one perception at once. Out of these various impressions, the most powerful, or that which has the greatest advantage to solicit my attention, overcomes and drives out the rest; or, which not less frequently happens, some idea of association, suggested by the last preceding idea, wholly withdraws my attention from every external object. It is probable however that this perception is imperceptibly modified by the miniature impressions which accompany it, just as we actually find that the very same ideas presented to a sick man take a peculiar tinge which renders them exceedingly different from what they are in the mind of a man in health. It has been already shown that, though there is nothing less frequent than the apprehending of a simple idea, yet every idea, however complex, offers itself to the mind under the conception of unity. The blending of numerous impressions into one perception is a law of our nature; and the customary train of our perceptions is entirely of this denomination.