A further ground of the suggestibility of the crowd is that prevalence of emotional excitement which was discussed in the foregoing pages. It is well recognised that almost any emotional excitement increases the suggestibility of the individual, though the explanation of the fact remains obscure. I have suggested that the explanation is to be found in the principle of the vicarious usage of nervous energy, the principle that nervous energy, liberated in any one part of the nervous system, may overflow the channels of the system in which it is liberated and re-enforce processes initiated in other systems. If this be true, we can see how any condition of excitement will favour suggestibility; for it will re-enforce whatever idea or impulse may have been awakened and made dominant by ‘suggestion.’ The principle requires perhaps the following limitation. Emotion which is finding outlet in well-directed action is probably unfavourable to all such ‘suggestions’ as are not congruent with its tendencies. It is vague emotion, or such as finds no appropriate expression in action, that favours suggestibility. The most striking illustrations of the greatly increased suggestibility of crowds are afforded by well-authenticated instances of collective hallucination, instances which, so long as we fail to take into account the abnormal suggestibility of the members of crowds, seem utterly mysterious, incredible, and super-normal. Again, the capacity of crowds to arrive at correct conclusions by any process of reasoning is apt to be diminished in another way by the exaltation of emotion to which, as we have seen, they are peculiarly liable. It is a familiar fact that correct observation and reasoning are hampered by emotion; for all ideas congruent with the prevailing emotion come far more readily to consciousness and persist more stably than ideas incongruent with it, and conclusions congruent with the prevailing emotion and desire are accepted readily and uncritically; whereas those opposed to them can hardly find acceptance in the minds of most men, no matter how simple and convincing be the reasoning that leads to them.