Other facts, too, seem to point to the importance of an element of the unknown. The common way of tickling a child is by running the fingers with discontinuous contact over the skin. Dr. L. Hill describes his mode of tickling in one case as running the fingers up the child’s arm _like a mouse_. This evidently brings in an element of _local_ uncertainty as well as of change. The effect is increased when, as frequently happens, there are pauses between the attacks of the fingers. The invasion of the skin-territory, like that of larger territories, is, it would seem, likely to be more effective when it has an element of unpredictableness. The uncertainty is, I believe, sometimes increased by half-voluntary variations in the direction and in the velocity of the tickling movements. Whether the fact communicated by Dr. L. Robinson, that a child is more ticklish when dressed than when undressed, is explained by the increased obscurity of the process in the former case, I am not sure. It is worth noting, however, that some of the areas said to be most ticklish, _e.g._, the armpits and the neck, are inaccessible to sight. I believe, too, that when a child gives himself up to the full excitement of tickling he makes no attempt to see what is going on. Now touches of unknown origin at places not closely observable have something of a disturbing character. A touch is always an attack, and has, so to speak, to be {62} condoned. This disturbing element I regard as an essential element in the experience: it goes along with the faintly disagreeable element of sensation, which, as we have assumed, is commonly, if not always, more or less clearly recognisable in the experience.[42] Yet it is certain that the disturbing effect (like the disagreeableness of the sensation) is limited. If the unknown bulks too largely and comes near the point of the alarming, the effect of laughter is wholly counteracted. This is a part of the explanation of the refusal of a child to be tickled by a stranger: for he knows here _too little_ of what is going to happen, and consequently is disposed to fear. Again, Dr. L. Hill informs me that “tickling a child unexpectedly and from an unseen quarter will not provoke laughter”: the element of surprise would seem in this case to be too great. Possibly the comparative difficulty of making a child laugh when naked may be explained by the increased apprehensiveness which goes with the defenceless state of nudity. The familiar fact that the readiness to laugh increases with practice, points to the same need of a certain comfortable assurance lying safely below the slight superficial apprehensions which are excited by the stimuli. All this suggests, that in order to call forth the glad response of laughter, we must secure a certain adjustment of stimulus to mental attitude. The tickling must fit in with a particular mood, the state of mind which makes enjoyment of fun not only possible but welcome.