The explanation of the success of suggestion in particular cases is to be sought in the emotional state of the subject. When I was the victim, as described above, my readiness to believe arose from my being accustomed to nocturnal interruptions when my patients were in trouble and also from my reliance on the hospital staff, my emotional state being one of expectation and confidence. If to these influences are added stronger emotional forces, such as wonder or terror, acceptance of suggestion is still easier, and when people assembled together are swayed by these feelings the Herd Instinct reaches its full strength and we have the ingredients for the manufacture of a collective delusion. There are many examples of strange and supernatural occurrences vouched for by masses of observers, and I see no reason to doubt the good faith of the historians. We all know how infectious is emotion and how hard it is for one man to remain unmoved when around him are others all under the influence of some excitement, and man always insists on finding reasons for his feelings or objects for his emotions. When wonder or terror are roused by the operation of the Herd Instinct, the individual, not knowing their origin, projects them externally and seeks an object for them. He is now ready to see or hear anything that will fit his emotions, and when an object is suggested he will speedily accept its existence as a reality. I will give some further examples of suggestion in varying degrees of strength. During the arrival of recently wounded men at a hospital in France, I was in a ward with two eminent members of my profession and another medical officer. As one man seemed bad the sister asked me to see him at once; his left arm was paralysed, and he had a wound on the head where in the brain beneath lies the 'motor area' of the left arm. Looking at the wound, which was obscured by hair and blood, I said, 'That's pulsating'; the two consultants and the other officer agreed with my observation, and appropriate treatment was recommended. The importance of pulsation lies in the fact that it is a sign of the exposure of brain substance, which pulsates strongly, and in this case it signified the presence of a hole in the skull which allowed the pulsation to appear; but in the operating theatre shortly afterwards the skull was found intact, and therefore pulsation had not been present. How did this joint error of observation arise? The combination of a gunshot wound of the head with a paralysed limb may occur in connection with a hole in the skull, and such penetrating wounds were common before the introduction of helmets. My unconscious had worked out the probabilities and led me to expect the signs of penetration; deceiving myself, by my confident manner I imposed my belief upon my colleagues, who had, I may assume, placed unjustified confidence in my reliability as an observer; and we all saw that which was not.