But in regard to the potency of experiencing libido the woman is generally inferior to the man. After three complete orgasms in one night even a young, strong woman will be completely enervated during the following day, and the woman who regularly experiences a daily orgasm, for a protracted period, will, no less than her active partner, fall a victim to neurasthenia after some time. _Intensity of libido._—The intensity of the pleasure varies in different individuals. With some the intensity reaches a very high degree. There are those who cry and bite at the height of orgasm.[N] On the other hand, there are individuals who scarcely have any lustful feelings, or, if capable of experiencing ant-orgastic libido, are lacking in the feeling of orgasm. Normally the intensity of libido and orgasm increases with the number and dignity of the points which produce the pleasure. The source of any kind of pleasure lies in the five senses. Of these the sense of touch is best adapted to provoke sexual pleasure. The tactile sense is the fundamental and generic sense from which all others take their rise, and by which they are verified. All the perceptions of our senses have to be supplemented by our tactile sense. The foetus, within the uterus, receives the first knowledge of its own individuality, of its own ego, by the tactile sense. In touching with its body the wall of the womb, it receives one impression, while in touching one part of its body with another part, it receives two impressions, at both points of contact. In this way it learns by repeated experience of the existence of bodies outside of his own. Thus it differentiates its own ego from the outside world by the sense of touch, at a time when the other senses could not as yet functionate. Touch is, therefore, the primary sense. The other senses are only modified tactile senses. Waves of sound touch the tympanic membrane of the ear: rays of light touch the retina in the eye; odors touch the nerve-endings of the olfactory nerve, and food the taste-papillae. The sense of touch is, therefore, the sense above all others to evoke pleasurable sensations. The touch of a soft and smooth surface pleases; that of a rough object displeases. Many people like to feel smooth objects, such as velvet. Others prefer smooth plants; yet others are fond of caressing animate beings, such as cats or dogs with a smooth fur. There are those who delight to touch the soft, smooth skin of babies. Now the softest and smoothest skin in the human body is found on the parts covered with a mucous membrane. The pleasurable sensations, therefore, increase when two individuals touch each other at parts covered with such a membrane. Hence, kissing on the lips, by reason of their covering, is accompanied by pleasure. This undoubtedly accounts for the origin of kissing.