One way of talking about what these experiences have in common is to classify them as "non-dual." Religions and philosophies that are "dual" see an irreconcilable gulf between the ordinary world and the enlightened state, between the soul and God, between the self and the world, or between consciousness and matter. Non-dual systems, on the other hand, teach that these boundaries are illusory. In India, entire religions have been built up around the premise that you can unify the individual self—with all of its problems of anxiety, fear, sadness, helplessness, and self-identification—with a greater, cosmic, divine, wise, and perfect whole that encompasses all of reality. Such traditions include most forms of Hinduism and Buddhism.

However, there is one thing that separates Buddhism from many other non-dual traditions, both in India and beyond. That is its emphasis on using subtraction rather than addition to describe its non-dual insights. A non-dual form of Hinduism, such as Advaita Vedanta, says that an individual's soul merges or unites with God. Buddhism, on the other hand, generally prefers to describe this insight in the negative. The self is inherently illusory, it insists. There is no God with which to merge. Instead, our true inner nature is a vast, still emptiness.

Buddhists tend to be much more precise about compassion than we usually are in everyday English. We commonly use the word in rather vague ways, to refer to a whole range of positive feelings. Buddhists, on the other hand, differentiate between four different positive mental states that might all be part of the ordinary English notion of compassion. Collectively known as the "immeasurable states of mind" (si wuliang xin) in Chinese or the "heavenly dwellings" (brahma vihara) in Pali, they are as follows:

• Loving kindness (metta in Pali): a feeling of universal friendliness, goodwill, and love toward all beings
• Empathetic compassion (karuna): a feeling of wanting to remove the suffering experienced by other beings.
• Altruistic joy (mudita): a feeling of joy at the happiness and success of other beings, untinged by jealousy or pride.
• Equanimity (upekkha): a feeling of tolerance, peace, and tranquility in the face of annoyances, including those caused by other beings.

These four immeasurables were understood to go together as a set in Indian culture long before the Buddha's time, and they appear in other Indian religious traditions as well. However, they were absorbed into Buddhism and came to be among its central ideas. The main idea behind the Buddhist discourses on this subject is that we can and should cultivate these positive states of mind. As is true with mindfulness or concentration, the brain can be trained in the immeasurables.

Part of the power of this kind of practice is that it has the ability to disrupt the loops of unkind, negative, and self-destructive thoughts toward ourselves and others that most of us normally hear playing inside our heads. If the thoughts floating down your mindstream are angry, for example, the perfect antidote may be to fill your mind up with loving kindness instead. If your thoughts tend toward cruelty or violence, why not replace them with empathetic compassion? If your thoughts often revolve around jealousy or greed, perhaps balance them with altruistic joy. If your mind is characteristically annoyed or perturbed by the people around you, then try to cultivate a little equanimity. Even if you recognize that this practice involves artificially creating positive affirmations and even if you know you are intentionally planting them in your head, it can still work. In fact, it can still work even if you don't actually feel anything while you are saying these words—even if your repetitions are completely mechanical.

The simple point I want to emphasize is that, just as the mind can be trained to concentrate more and more through the practice of mindfulness meditation, it can also be trained to become more and more kind, compassionate, and even-keeled through meditation practice focused on the immeasurables. There isn't any magic woo-woo going on here. From what we know about the neuroplasticity of the brain, it is not at all surprising that we can transform anger, anxiety, and self-concern into gentleness, calm, and generosity toward others if we are willing to commit the time to the practice of reprogramming ourselves.

... talk about subtle energies or winds is at its core an elaborate cluster of metaphors that clearly describes—and tunes you into—how the sensations in your body change in response to your mind. I don't think there's necessarily anything mystical or supernatural going on here. Rather, I see it as a culturally specific way of describing a common everyday experience that all human beings share—and that any of us can notice if we pay attention.

...

... the main point Buddhism is trying to get across is that it’s helpful for your health to improve your interoception, your attention to what's going on inside your body. When you pay more attention to what's going on inside, you can see how the mind triggers negative reactions in the body and the body responds to the mind. Once you can observe this process in real time, you can learn ways to intervene in order to bring yourself a bit of relief.

... in the Zen practice of "just sitting," one keeps one's attention wide open, choicelessly accepting anything that comes into one’s awareness, chirping birds and all. Far from tuning them out, it seems that sounds, sights, and other perceptions become heightened by this kind of practice. You start to perceive things more crisply, and everything can seem more beautiful than ever. This enhanced perception may be one reason that Zen masters have frequently celebrated the natural world and have lovingly dwelt on small details in the environment.

You'll notice the suffix -do (pronounced something like the English word “doe”) on the end of both kyudo and chado, as well as on the word for calligraphy, shodo, the word for flower arrangement, kado, and on the words for many other traditional Japanese arts. On its own, do means "path" or "way." When you add do to an activity in Japanese, you are implying that it is no longer a conventional activity but an intentional practice. You're no longer just doing calligraphy or shooting arrows or arranging flowers or drinking tea. Rather, you have elevated these activities to rituals with great significance. It's now a matter of form arising spontaneously out of emptiness, an expression of your Buddha-nature, an enactment of the non-dual union of ultimate truth and conventional truth.

To become a do, a practice requires years—decades, in fact—of training and a very high level of expertise. The Zen emphasis on naturalness does not mean that actions are unplanned—the whole idea is that it takes intricate focus and exactitude to execute a simple spontaneous action. With the right attention to detail and the right mindset, however, anything at all can be a do. At least that's my interpretation. If you remain grounded in emptiness, allow the actions to spontaneously occur, and are unattached to the outcomes, your whole life can be a do. Every mundane action can be transmuted into an expression of your Buddha-nature. What would it be like to live life from that place?

For most Buddhists, it's not only all beings but also all objects that are intricately interconnected. The Vietnamese monk Thich Nhat Hanh calls this interrelatedness "interbeing." In an essay titled "What is Interbeing?," he uses the example of a piece of paper to illustrate the idea. A piece of paper, he says, could not exist independently of all the other things with which it is connected. In order for the paper to arrive in your hand, we need the sun, clouds, rain, trees, loggers, paper mills, warehouses, stores, and everything else along the way. Each of those links in the chain, in turn, can't exist independently of all sorts of additional interconnected factors—the logger, for example, wouldn't exist if it wasn't for his parents and grandparents, or all the food he has eaten throughout his life.

There is an ancient Indian metaphor called Indra's net, or Brahma's net, that makes this same point in a more visual way. This vivid image is found in both the Flower Ornament Scripture and the Brahma's Net Scripture, two Mahayana texts that have been highly influential in East Asia. The metaphor hinges on the image of a massive net belonging to the god Indra or Brahma, which is made completely of jewels. The net extends infinitely in all directions so that it covers the entire cosmos, and the jewels are so clear and so bright that each one reflects all the others simultaneously. Thus, if you look at any one jewel, you see the reflection of the entire universe all at once.

Both the example of the paper and the metaphor of the jeweled net make the same point: Begin from any object, no matter how simple—whether a piece of paper, a cup of coffee, your own fingernail, or a discarded piece of trash on the side of the road—and follow the connections of interbeing outward. Before you know it, you'll realize that, in that seemingly mundane thing, you are looking at a reflection of the entire cosmos. All objects are inseparable from each other, and everything is constantly arising and existing together in one interconnected whole.

The mystical experience of the oneness of everything is emphasized more in Mahayana forms of Buddhism than in Theravada, and particularly in Zen and Vajrayana. Having this kind of realization even for a moment can have a profound, irreversible, and liberating effect on how you perceive yourself and the world. You may discover a bottomless well of compassion that you can effortlessly share with all beings, because you see them all as inseparable from yourself. You may experience all things as interlinked entities and be overwhelmed with gratitude for how each one plays its own unique role within the grand unfolding of the universe. You may experience all things, including yourself, as a single field of experience that has no boundaries and is always transforming and giving birth to something new. You may be struck by the sheer beauty of nature or of everyday objects, seeing each as a miraculous expression of its own "suchness" (tathata) that couldn't ever be anything other than the way it perfectly exists right now. Some of the most mystical descriptions of this kind of realization say that the whole universe reveals itself to be the Dharma body of the Buddha. That is to say, the entire universe is experienced as one seamless, alive, wholeness that is Awakeness itself.