The particular emotions which are invoked by pain to reinforce the sexual impulse are more especially anger and fear, and, as we have seen, these two very powerful and primitive emotions are--on the active and passive sides, respectively--the emotions most constantly brought into play in animal and early human courtship; so that they naturally constitute the emotional reservoirs from which the sexual impulse may still most easily draw. It is not difficult to show that the various forms in which "pain"--as we must here understand pain--is employed in the service of the sexual impulse are mainly manifestations or transformations of anger or fear, either in their simple or usually more complex forms, in some of which anger and fear may be mingled. We thus accept the biological origin of the psychological association between love and pain; it is traceable to the phenomena of animal courtship. We do not on this account exclude the more direct physiological factor. It may seem surprising that manifestations that have their origin in primeval forms of courtship should in many cases coincide with actual sensations of definite anatomical base today, and still more surprising that these traditional manifestations and actual sensations should so often be complementary to each other in their active and passive aspects: that is to say, that the pleasure of whipping should be matched by the pleasure of being whipped, the pleasure of mock strangling by the pleasure of being so strangled, that pain inflicted is not more desirable than pain suffered. But such coincidence is of the very essence of the whole group of phenomena. The manifestations of courtship were from the first conditioned by physiological facts; it is not strange that they should always tend to run _pari passu_ with physiological facts. The manifestations which failed to find anchorage in physiological relationships might well tend to die out. Even under the most normal circumstances, in healthy persons of healthy heredity, the manifestations we have been considering are liable to make themselves felt. Under such circumstances, however, they never become of the first importance in the sexual process; they are often little more than play. It is only under neurasthenic or neuropathic conditions--that is to say, in an organism which from acquired or congenital causes, and usually perhaps both, has become enfeebled, irritable, "fatigued"--that these manifestations are liable to flourish vigorously, to come to the forefront of sexual consciousness, and even to attain such seriously urgent importance that they may in themselves constitute the entire end and aim of sexual desire. Under these pathological conditions, pain, in the broad and special sense in which we have been obliged to define it, becomes a welcome tonic and a more or less indispensable stimulant to the sexual system.