We can only judge by the circumstances under which actions are performed, whether they are due to instinct, or to reason, or to the mere association of ideas; this latter principle, however, is intimately connected with reason. A curious case has been given by Professor Möbius, of a pike, separated by a plate of glass from an adjoining aquarium stocked with fish, and who often dashed himself with such violence against the glass in trying to catch the other fishes, that he was sometimes completely stunned. The pike went on thus for three months, but at last learned caution, and ceased to do so. The plate of glass was then removed, but the pike would not attack these particular fishes, though he would devour others which were afterward introduced; so strongly was the idea of a violent shock associated in his feeble mind with the attempt on his former neighbours. If a savage who had never seen a large plate-glass window, were to dash himself even once against it, he would for a long time afterward associate a shock with a window-frame; but, very differently from the pike, he would probably reflect on the nature of the impediment, and be cautious under analogous circumstances. Now with monkeys, as we shall presently see, a painful or merely a disagreeable impression, from an action once performed, is sometimes sufficient to prevent the animal from repeating it. If we attribute this difference between the monkeys and the pike solely to the association of ideas being so much stronger and more persistent in the one than the other, though the pike often received much the more severe injury, can we maintain in the case of man that a similar difference implies the possession of a fundamentally different mind?