"Now, since I do not live in his skin, there remains only the knowledge, that is, the mental picture, I have of him, as the possible means where- by I can so far identify myself with him, that my action declares the difference to be practically effaced. The process here analyzed is not a dream, a fancy floating in the air ; it is perfectly real, and by no means infrequent. It is, what we see every day, the phenomenon of Compassion ; in other words, the direct participation, independent of all ulterior considerations, in the sufferings of another, leading to sympathetic assistance in the effort to prevent or remove them ; whereon in the last resort all satisfaction and all well-being and happiness depend. It is this Compassion alone which is the real basis of all voluntary justice and all genuine loving-kindness. Only so far as an action springs therefrom, has it moral value ; and all conduct that proceeds from any other motive whatever has none. When once compassion is stirred within me, by another's pain, then his weal and woe go straight to my heart, exactly in the same way, if not always to the same degree, as otherwise I feel only my own. Consequently the difference between myself and him is no longer an absolute one.
No doubt this operation is astonishing, indeed hardly comprehensible. It is, in fact, the great mystery of Ethics, its original phenomenon, and the boundary stone, past which only transcendental speculation may dare to take a step. Herein we see the wall of partition, which, according to the light of nature (as reason is called by old theologians), entirely separates being from being, broken down, and the non-ego to a certain extent identified with the ego."
Arthur Schopenhauer, On the Basis of Morality Part III pg. 170-171