I’ve found that the hardest Buddhist concepts to understand are those which predate Buddhism in one way or another. One of these is the Buddha’s teaching on the four Brahma-viharas: metta, karuna, mudita, and upekkha.
In the Pali suttas they are almost always mentioned as a set without additional descriptions, such that it is hard to know where each begins and ends.
Thanissaro Bhikkhu’s article Head and Heart Together: Bringing Wisdom to the Brahma-viharas does a really great job of explaining the Brahma-viharas and their interrelationships in a way this hapless practitioner can understand:
Of these four emotions, goodwill (metta) is the most fundamental. It’s the wish for true happiness, a wish you can direct to yourself or to others. […] The next two emotions in the list are essentially applications of goodwill. Compassion (karuna) is what goodwill feels when it encounters suffering: It wants the suffering to stop. Empathetic joy (mudita) is what goodwill feels when it encounters happiness: It wants the happiness to continue. Equanimity (upekkha) is a different emotion, in that it acts as an aid to and a check on the other three. When you encounter suffering that you can’t stop no matter how hard you try, you need equanimity to avoid creating additional suffering and to channel your energies to areas where you can be of help.
This article did not suddenly teach me the error of my ways or cause me to exclaim, “Eureka! I should treat people well! My random face-punching days are over!” The article instead was refreshing in it structure and specificity.
There is a tendency, I feel, among some Buddhist writers and teachers to show how there are infinite manifestations of compassion, each with its wildly varying details. I’m sure this is done to show the wonder and grandeur of compassion, but as someone looking to be more compassionate and understand how compassion works, being about everything is not much different from being about nothing.
The article is full of well-crafted distinctions written in clear language. Another favorite of mine comes from the title: the disparity between the head and the heart. This proliferation of this distinction is a pet peeve of mine, since all too often we speak of the contest between the head and the heart as if they were two separate warring impulses rather than ambivalence born from the disparity between what we want and the way we know things are. Thanissaro Bhikkhu defines the disparity thusly:
If we think of the heart as the side of the mind that wants happiness, the head is the side that understands how cause and effect actually work. If your head and heart can learn to cooperate — that is, if your head can give priority to finding the causes for true happiness, and your heart can learn to embrace those causes — then the training of the mind can go far.