Shodo Harada Roshi:

In Buddhism, its often said that humans’ Original Mind, that Mind we have at birth, is like a clear mirror, pure and uncluttered, without shape, form, or color, with nothing in it whatsoever. If something comes before it, the mirror reflects it exactly, but the mirror itself gives birth to nothing. If what has been reflected leaves, its image disappears, but the mirror itself loses nothing. Within the mirror there is no birth, no death. No matter how dirty a thing that is reflected might be, the mirror doesn’t get dirty, nor does it become beautiful because something beautiful is reflected in it. Just because additional things are reflected, that doesn’t mean anything increases in the mirror itself, nor does anything ever decrease when fewer objects are reflected. A mirror is without increase or decrease.

Humans’ pure Original Nature is just this. Without shape, form, or color; without birth and death; not clean or dirty; not increasing or decreasing; not male or female; not young, not old; not intelligent, not stupid; not rich, not poor. There are no words, no explanation possible, no description that will apply here, only a pure mirror-like base. This is humans’ true quality; this is an actual experience. From our zazen (sitting meditation), cut all nen (mind-instants), dig down completely to the source of those nen—dig, dig, dig until we reach the place where the human character has been totally cleared. When the source point is reached, this state of Mind can be touched.

This clear human character, which is like a mirror, can accept and receive everything, but nothing that is reflected can get stuck to this mirror. It reflects everything exactly as it is, but the mirror itself stays untouched. This mirror-like Mind has no sense of "that’s me" or "that’s him, not me." It has no dualism; it makes no distinctions like that. At that true base, there actually is no differentiation between self and others. The world that is reflected in—reflected by—that mirror is not one of self and other; it has no such separation, it accepts everything as one unified whole. From the origin there is only one world, with no division into "my" world and "your" world.

To understand this as an actual fact with your own experience is the wisdom of the Buddha. From there arises the functioning of the human Mind that naturally feels another’s pain as one’s own pain, feels another’s joy as one’s own joy. A warm, encompassing Mind naturally arises from this wisdom and experience. That is what is called the compassion of the Buddha.

If we can realize the source point of our human character, then naturally all of the world becomes One. Not divided, it is encountered as one unified Whole, a great, expansive, and huge world of One. Wisdom works here and humans’ joy, suffering, and sadness become our own joy, suffering, and sadness. It is not somebody else’s joy; it is one’s very own joy as well. This is how a warm, all-encompassing Mind becomes naturally revealed and serves as the source of our action. Simply put, this is what the Buddha meant when he said, "Seek the light within yourself."