When the overwhelming importance of the suggestibility of man is recognized our first effort should be to obtain exact numerical expressions of it. This is not the place to attempt any exposition of the directions in which experiment should proceed; but it may be stated that what we want to know is, how much suggestion can do in the way of inducing belief, and it may be guessed that we shall ultimately be able to express the force of suggestion in terms of the number of undifferentiated units of the herd it represents. In the work that has already been done, chiefly by Binet and by Sidis, the suggestive force experimented with was relatively feeble, and the effects consequently were rendered liable to great disturbance from the spontaneous action of other forces of suggestion already in the mind. Sidis, for example, found that his subjects often yielded to his suggestions out of “politeness”; this source of difficulty was obviously due to his use of pure individual suggestion, a variety which theory shows to be weak or even directly resisted. The next feature of practical interest is connected with the hypothesis, which we attempted in the former article to demonstrate, that irrational belief forms a large bulk of the furniture of the mind, and is indistinguishable by the subject from rational verifiable knowledge. It is obviously of cardinal importance to be able to effect this distinction, for it is the failure to do so which, while it is not the cause of the slowness of advance in knowledge, is the mechanism by which this delay is brought about. Is there, then, we may ask, any discoverable touchstone by which non-rational opinion may be distinguished from rational? Non-rational judgments, being the product of suggestion, will have {44} the quality of instinctive opinion, or, as we may call it, of belief in the strict sense. The essence of this quality is obviousness; the truth held in this way is one of James’s “_a priori_ syntheses of the most perfect sort”; to question it is to the believer to carry scepticism to an insane degree, and will be met by contempt, disapproval, or condemnation, according to the nature of the belief in question. When, therefore, we find ourselves entertaining an opinion about the basis of which there is a quality of feeling which tells us that to inquire into it would be absurd, obviously unnecessary, unprofitable, undesirable, bad form, or wicked, we may know that that opinion is a non-rational one, and probably, therefore, founded upon inadequate evidence.