These and other familiar facts point to the conclusion that the laughter excited by tickling is not a net effect of the sensory stimulation. It is no doubt broadly determined by the characteristics of the sensations. Intensely disagreeable ones would certainly not call forth the laughing response. But the determining conditions include, in addition to a sequence of sensations, _a higher psychical factor_, namely, an apperceptive process or assignment of _meaning_ to the sensations. This conclusion is borne out by the fact that the laughter-reaction occurs first of all (to give the earliest date) in the second month—presumably in the second half of this month. The presence of such a psychical factor is more strongly supported by the fact, already referred to, that the reaction does not occur in the first three months save when mental agencies co-operate; and that throughout the ticklish period an exactly similar process of titillative stimulation applied to the same area of the {60} skin will now produce laughter, now fail to do so, according to the varying mood of the child.[39] That the interpretation of the sensation is the decisive element in eliciting laughter may, I think, be seen by a simple experiment which any reader who is ticklish may carry out upon himself. The next time he happens to have a subjective, creepy skin sensation, he will find that he can bring on either laughter or a very different state of feeling by adopting one of two ways of mentally envisaging what is happening. The merest suggestion of an invading parasite suffices, I believe, to set up a mental state which completely inhibits the impulse to laugh. We may now seek to assign with more precision the mental conditions which induce the mode of apperception favourable to laughter. Beginning with the “objective” characteristics, those which reside in the tickling experience itself, we may observe how much apprehension of meaning has to do with the “funniness” of the experience. It is to be noticed at the outset that when we are tickled there is _an element of the unknown_ in the process. This seems to have been recognised by Darwin when he laid emphasis on the fact that the more ticklish parts are those rarely touched, at least on small areas, and, one may add, lightly.[40] The familiar fact that one cannot tickle oneself points to the same conclusion. A person who tries to do this knows too much about what is going on. Dr. Ch. Richet observes, however, that one can tickle oneself _by means of a feather_; and he, as I think {61} rightly, explains this apparent exception by saying that in the attempt to tickle oneself with the finger, the double sensation, of the finger and the part tickled, seems to inhibit the effect, whereas, when the feather is interposed this obstacle is eliminated.[41]