According to the common assumption, laughter, in ordinary cases, is excited by some provocative, to speak more precisely, by some sense-presentation, or its representative idea, such as a “funny” sensation, the sight of a droll human figure, or a quaint fancy. Yet we must not assume that such an initial presentation occurs in all cases. As is implied in what has been said above about the laughter of “good spirits,” and as we shall see more clearly presently, there are cases where laughter takes on the appearance of a spontaneous or “automatic” group of movements. 1. It may be well, however, to begin our inquiry by touching on those varieties of laughter in which the action of a sense-stimulus is apparent. And it will be convenient to select a form of distinctly provoked laughter in which the intellectual processes play only a subordinate part. The effect of tickling is clearly of this kind, and as one of the {51} simplest modes of exciting laughter it seems to claim our first attention here. Since, moreover, it is the mode of exciting laughter of which our knowledge has been rendered in a measure precise by means of experiment, I propose to deal with it at some length. The experience of being tickled is best described in its entirety as a sensational reflex; that is to say, a motor reaction on a process of sensory stimulation which produces a well-marked variety of sensation. To speak of titillation as if it were merely the production of a certain kind of sensation is unscientific. It involves the excitation of certain movements, and where these are not forthcoming we must infer, either that the sensory part of the process is defective, or that the motor impulse is inhibited in some way. The stimulation in this case is, as we all know, a light tactile one. The agent commonly applied is the finger or a still softer body, such as a feather. The mode of contact is light, or at least does not commonly rise to the point of heavy pressure. The manner of contact is usually intermittent, the finger or fingers giving a series of short and staccato impacts. Movements of the fingers from point to point commonly accompany the series of contacts. In some cases, however, a single light touch, or even a continuous touch with movement from point to point, may suffice to induce the proper effect.