Now these facts suggest that even those varieties of tickling which produce a sensation having a well-marked disagreeable tone may excite the response of laughter. The tickling of the sole of the foot not only provokes laughter in an infant; it tends to do so, I believe, in an adult, who may at the same time express his dislike of the sensation by a grimace. It seems impossible then to conclude that the laughter which arises from tickling is a mere expression of the pleasure-tone of a sensational process. Even if we supposed that in all cases the sensations were preponderantly agreeable, it would still be impossible to account for the energy of the reaction by the intensity of the sensuous enjoyment experienced. That we have not to do here merely with the effect of agreeable stimulation is shown by the fact that when a child laughs under, and is said to enjoy, a process of titillation, _the laughter is accompanied by defensive movements_. When, for example, a child is tickled on its back, it will, says Dr. Robinson, “wriggle about, fencing with its arms and dodging the attacks of its playmate . . . _laughing all the time_ {59} _with open mouth and teeth fully displayed_”. This surely suggests that the laughter is not merely the result of an agreeable sensation, but rather of a complex mental state, in which the agreeable and disagreeable elements of sensation appear to play only a secondary _rôle_. Nor again does it seem as if the mere transition from an agreeable to a disagreeable sensation, or the reverse process, would account for the laughter of tickling. A person highly sensitive to the effect of tickling can imitate the process by movements of his own fingers, and produce quite similar sensations of varying feeling-tone _without experiencing the faintest impulse to laugh_. Again we know that other experiences, such as scratching a sore place when it is healing up, involve an alternation of moments of agreeable and disagreeable feeling-tone, and yet are not provocative of laughter.