The section "Selfishness and Compassion" from "The Oxford Reader: Schopenhauer" states:
If every person's psychological structure contains a component of compassion, what remains? Schopenhauer's complete answer is as follows:
The three basic ethical motives of humans—selfishness, malice, and compassion exist in different, vastly varying proportions in each individual. These motives will act upon individuals according to these proportions and lead to their behavior. (On the Basis of Morality, 192)
Selfishness is "immense" and "natural":
Each individual, almost negligible in a vast and boundless world, makes themselves the center of the world, placing their own survival and well-being above all else. In fact, from a natural perspective, they are prepared to sacrifice everything for this; to allow their minuscule existence to last just a little longer, they would be willing to destroy the world. This tendency is selfishness, which is essential for all things in nature. (The World as Will and Representation, Volume 1, 332)
Selfishness so greatly "overrides the world" (On the Basis of Morality, 132), that without the constraints of law represented by the state machinery, individuals would be drawn into a war of all against all (On the Basis of Morality, 133). All of this means that actions motivated by the welfare of others should not only be exceedingly rare but also impossible due to their contradiction with our nature. Schopenhauer had to admit that compassion is an enigma in ethics. The only thing he could say is that compassion is a primitive characteristic that counters selfishness, existing as a pure fact within us. But how compassion can "reside within human nature" (On the Basis of Morality, 149) is a profound mystery, as humans are a natural expression of the will to live's selfishness.
So let me explain, how can humans have a sense of compassion? There is a saying, "When the rabbit dies, the fox mourns; creatures grieve for their kind," which does not mean that a fox will truly mourn a dead rabbit, but rather metaphorically refers to the sadness caused by the death of companions or kindred. Humans have a sense of compassion because they see in their kind the potential for experiences that could befall themselves; their imagination allows them to place themselves in the situations faced by others, thus experiencing mental anguish, which leads to the emergence of compassion, feeling sorrow for the suffering of others. This is similar to how we immerse ourselves in movies or artistic works, identifying with a character and sharing in their joys and sorrows; we say this is due to our capacity for empathy or the ability to take on another's role. The premise of this ability is that humans possess imagination. Therefore, what humans pity is actually an imagined version of themselves.
Reflections on reading "The Oxford Reader: Schopenhauer"