Touch, as has already been remarked, is the least intellectual of the senses. There is, however, one form of touch sensation--that is to say, ticklishness--which is of so special and peculiar a nature that it has sometimes been put aside in a class apart from all other touch sensations. Scaliger proposed to class titillation as a sixth, or separate, sense. Alrutz, of Upsala, regards tickling as a milder degree of itching, and considers that the two together constitute a sensation of distinct quality with distinct end-organs, for the mediation of that quality.[5] However we may regard this extreme view, tickling is certainly a specialized modification of touch and it is at the same time the most intellectual mode of touch sensation and that with the closest connection with the sexual sphere. To regard tickling as an intellectual manifestation may cause surprise, more especially when it is remembered that ticklishness is a form of sensation which reaches full development very early in life, and it has to be admitted that, as compared even with the messages that may be sent through smell and taste, the intellectual element in ticklishness remains small. But its presence here has been independently recognized by various investigators. Groos points out the psychic factor in tickling as evidenced by the impossibility of self-tickling.[6] Louis Robinson considers that ticklishness "appears to be one of the simplest developments of mechanical and automatic nervous processes in the direction of the complex functioning of the higher centres which comes within the scope of psychology,"[7] Stanley Hall and Allin remark that "these minimal touch excitations represent the very oldest stratum of psychic life in the soul."[8] Hirman Stanley, in a somewhat similar manner, pushes the intellectual element in ticklishness very far back and associates it with "tentacular experience." "By temporary self-extension," he remarks, "even low amoeboid organisms have slight, but suggestive, touch experiences that stimulate very general and violent reactions, and in higher organisms extended touch-organs, as tentacles, antennæ, hair, etc., become permanent and very delicately sensitive organs, where minimal contacts have very distinct and powerful reactions." Thus ticklishness would be the survival of long passed ancestral tentacular experience, which, originally a stimulation producing intense agitation and alarm, has now become merely a play activity and a source of keen pleasure.[9]