Now pain, taken in its profoundest sense, consists of two essential elements: a shock to the conscious self, the _ego_, in the first place; and, in the second place, the prolongation of the shock. If the self is not distinctly conscious, if it does not go so far as to assert itself by the separation of that self from the external world, we cannot say that pain is possible. The _ego_ never asserts itself with so much force as under a very painful impression. So that among beings whose reactions are mechanical, automatic, governed by other forces than by the assertion of the self and a freely deliberate will, pain becomes so indistinct, so confused, that it probably does not exist in the strict psychological sense at all. The greatest philosopher of modern times, Descartes, imagined a system of machine-animals; this idea has been turned into ridicule by the ignorant, but nevertheless we are almost forced to return to it when we dive to the bottom of reflex movements. Now, if we are able to admit that there is a vague consciousness of the selfhood among superior animals, such as the mammalia and birds, this consciousness, as far as concerns the inferior vertebrata, is most certainly extremely hazy, if, indeed, it exists at all. I have difficulty in conceiving that a frog is able to ponder over its _ego_, assert its existence in presence of the external world, and say or think, I SUFFER. No being suffers unless he is able to think that he suffers, and meditate on his suffering. To suffer means to have consciousness; and as far as it is permissible for a man to picture to himself the sensations of a frog, I should say that the frog has no consciousness of suffering. Even as regards the more highly developed vertebrata, such as birds, rabbits, and guinea-pigs, suffering is probably of a very obscure nature. It is not enough to say that an animal suffers because we see him animated by the contortions and reactions of defence. The new-born infant, which has neither intelligence nor memory nor consciousness, is probably incapable of real conscious suffering, nevertheless it screams and cries when it is hungry or when it is pricked. But these screams and tears do not suffice to allow us to affirm that the child is suffering real pain. It is a nervous excitation which is translated by the reactions of defence; it is not the conscious assertion of an _ego_ which has been painfully perturbed.