Empathy is one of human beings’ highest qualities. Empathy is the root of most of the behaviour that we associate with “goodness.” It’s the root of compassion and altruism, self-sacrifice and charity. Conversely, a lack of empathy is the root of most destructive and violent behaviour - in fact, everything that we associate with “evil.” A lack of empathy with victims makes crime possible. A lack of empathy with other human groups makes warfare possible. A lack of empathy enables psychopaths to treat other human beings callously, as objects who have no value except as a means of satisfying their desires.
You can think of empathy as a channel which connects human beings to one another. Compassion is what flows through the channel, and the result of this compassion is altruism - selfless action which aims to alleviate the suffering or further the development of others.
What is perhaps less well-recognised is that there are two fundamentally different kinds of empathy. The first is what might be called “shallow empathy.” In fact, this is the most common definition of empathy, as the ability to “put yourself in someone else’s shoes,” or see the world through someone else’s eyes, or to read their emotions. In other words, empathy is seen as a cognitive ability, along the same lines as the ability to imagine future scenarios or to solve problems based on previous experience. As the psychologist Paul Gilbert points out, empathy in this sense doesn’t necessarily imply "goodness." According to Gilbert, empathy is what makes torture possible. Without empathy, a torturer would have no concept of the suffering he is causing. Because he can “put himself in another person’s shoes” he knows that he is causing pain.
The second type of empathy is what I call “deep empathy.” This is more than just a cognitive ability. It’s the ability not just to imagine but to actually feel what other people are experiencing. It’s the ability to actually enter the “mind space” of another person so that you can sense their feelings and emotions. In a sense, your identity merges with theirs. The separateness between you and them fades away. Your "self-boundary" melts away, so that in a sense – or to an extent – you become them.
If you experience “deep empathy” then it becomes impossible to inflict pain or suffering on other people, at least intentionally. In deep empathy, you recoil from other people’s pain in the same way that you recoil from your own pain. You are reluctant to harm them in the same way that you are reluctant to harm yourself.
One of the interesting things about these two types of empathy is that they are not necessarily related. This is clear from Paul Gilbert’s example of torture. With deep empathy, torture is impossible. At the same time, it’s possible that a person may possess “deep empathy” and lack “shallow empathy” to a degree. That is, they may have a strong ability to “feel with” other people and sense their suffering, and at the same time not be particularly adept at reading emotional signals, or seeing from another person’s perspective. (I admit that this may be true of myself!) This is possible because these two types of empathy have different sources: one is cognitive, the other is affective. (As well as the terms "deep" and "shallow" empathy, they could be referred to as "cognitive" and "affective" empathy.)
The major moral teaching of all world religions is “treat other people as you would like to be treated yourself.” In Judaism, this is expressed in the saying “That which is hateful to you, do not do to your fellow. That is the whole Torah.” In Hinduism, the concept of daya means that we should attempt to alleviate the sufferings of all other human beings - including strangers and enemies - because they are part of our own being. And this is a moral imperative which stems directly from “deep empathy” - and one which stems from the highest part of our nature.