Zen often uses the analogy of a mirror to explain the function and nature of the mind. A mirror has no color of its own; it simply reflects the colors that appear in front of it. When red appears, the mirror turns red. So when an object such as a a cloud or flower appears in our consciousness, a clear mind does not embellish or add to them. It simply reflects their content.
Similarly, when anger appears, the true function of the mind is simply to reflect the emotion. Since the mind is clear with no nature of its own, like the mirror, it
becomes red when red appears. This can be seen in the Zen approach to emotions. Teachers often instruct students, when they are angry, to be
fully angry; allow no gaps between oneself and the anger. In other words,
become the emotion (which does not mean
act upon it) so there is no separation between one's mind and the experience.
To continue the mirror analogy: this means that, in a sense, the mirror's surface
is red. The implication is that, since Buddha is Mind, whatever appears in the mind
is it. It's all the truth. This represents the predominant approach to mind found in Zen schools today.
There is, however, another, slightly different metaphor--the mani pearl. This pristine jewel, while having no color its own, reflects the colors that appear in front of it, the same way that the mirror does. However, while the mirror
becomes red, the mani pearl maintains its luminosity. In other words, it does not
become the color it reflects.
The mani pearl remains pure, clear, serene, and luminous; for it never fully
becomes the colors that it reflects. Its surface simply adopts
the colors, without becoming them.
These two analogies reflect (pardon the pun) very different positions in respect to practice and soteriology. For instance, the mirror implies that there is no intrinsic nature to the mind; therefore, the mind
becomes its content. While the mani pearl metaphor argues that the mind does possess a nature of its own that allows the mind to resists
becoming its contents. That nature is clear, vast luminosity.
The former tends to exemplify dependent origination (the teaching of emptiness and interdependence); the latter, nature origination, which advocates that the Absolute (personified by the mind) is the fundamental basis and source of all phenomena.
I am currently working on a book about nature origination and its implications on Zen practice. More on that soon.