Instinct is not a mechanism of brain or mind implanted in living beings by nature; for, if it were, then instinctive action without any, even unconscious, activity of mind, and with no conception concerning the purpose of the action, would be executed mechanically, the purpose having been once for all thought out by Nature or Providence, which has so organised the individual that it acts henceforth as a purely mechanical medium. We are now dealing with a psychical organisation as the cause instinct, as we were above dealing with a physical. A psychical organisation would be a conceivable explanation and we need look no farther if every instinct once belonging to an animal discharged its functions in an unvarying manner. But this is never found to be the case, for instincts vary when there arises a sufficient motive for varying them. This proves that special exterior circumstances enter into the matter, and that these circumstances are the very things that render the attainment of the purpose possible through means selected by the instinct. Here first do we find instinct acting as though it were actually design with action following at its heels, for until the arrival of the motive, the instinct remains late and discharges no function whatever. The motive enters by way of an idea received into the mind through the instrumentality of the senses, and there is a constant connection between instinct in action and all sensual images which give information that an opportunity has arisen for attaining the ends proposed to itself by the instinct. The psychical mechanism of this constant connection must also be looked for. It may help us here to turn to the piano for an illustration. The struck keys are the motives, the notes that sound in consequence are the instincts in action. This illustration might perhaps be allowed to pass (if we also suppose that entirely different keys can give out the same sound) if instincts could only be compared with _distinctly tuned_ notes, so that one and the same instinct acted always in the same manner on the rising of the motive which should set it in action. This, however, is not so; for it is the blind unconscious purpose of the instinct that is alone constant, the instinct itself—that is to say, the will to make use of certain means—varying as the means that can be most suitably employed vary under varying circumstances.