In piano playing, the stimulus is the notes written in their particular places on the staff. Not only must the position of the notes on the staff be taken into account, but also many other things, such as sharps and flats, and various characters which give directions as to the manner in which the music is to be played. The striking of the notes in the proper order, in the proper time, and with the proper force, is the response. In typewriting, the stimulus is the copy, or the idea of what is to be written, and the response is the striking of the keys in the proper order. Speaking generally, we may say that the stimulus is the force or forces which excite the sense organs, and thereby, through the nervous system, bring about a muscular response. This is the ordinary type of action, but we have already indicated a different type. In speaking of typewriting we said the stimulus might be either the copy or ideas. One can write from copy or dictation, in which the stimulus is the written or spoken word, but one can also write as one thinks of what one wishes to write. The latter is known as _centrally initiated action_. That is to say, the stimulus comes from within, in the brain, rather than from without. Let us explain this kind of stimulation a little further. Suppose I am sitting in my chair reading. I finish a chapter and look at my watch. I notice that it is three o'clock, and recall that I was to meet a friend at that time. The stimulus in this case is in the brain itself; it is the nervous activity which corresponds to the idea of meeting my friend. If we disregard the distinction between mind and body, we may say that the stimulus for a response may be an idea as well as a perception, the perception arising from the immediate stimulation of a sense organ, and the idea arising from an excitation of the brain not caused by an immediate stimulation of a sense organ. =Instincts and Habits.= In human action it is evident that there is always a stimulus to start the nerve-impulse which causes the action. If we make inquiry concerning the connection between the stimulus and response; if we ask how it has come about that a particular stimulus causes a particular response rather than some other possible response, we find two kinds of causes. In one case the causal connection is established through heredity; in the other, the causal connection is established during a person's lifetime through training.