It is next to impossible for the average person or even the average Buddhist to divine the difference between a mental image of pure Mind and actual pure Mind, itself. One reason for the difficulty is the average person or Buddhist practitioner has never been totally free of mental images.
Our sense of pure Mind or even Buddha-nature is really the reflection of the unconditioned pure Mind on the conditioned self-conscious intellect. An apt analogy is the moon in the sky, i.e., the real moon, and the moon reflected on the surface of the water. When the Buddha speaks of pure Mind, for us, this is the moon on the water which we wrongly imagine to be the same as the moon in the sky.
If the reflected moon happens to appear in a shallow puddle of water, then in a day it disappears, the real moon is still in the sky. Likewise when worldlings reflect on pure Mind or Buddha-nature, this is like the moon reflected in a shallow puddle of water. Within a few seconds the Buddha-nature in the beginner’s mind completely disappears who then thinks, wrongly, that the Buddha-nature is impermanent. As the reader can see this deluded person believes the real eternal Buddha-nature, of which the Buddha’s discourses speak, has disappeared when, in truth, only the conditioned Buddha-nature which is based on this person’s conditioned consciousness has disappeared.
Such deluded thinking dominates modern Buddhist thought which believes that all of Buddhism is centered on a puddle of water, so to speak. For those engaged in this kind of thinking, everything is transitory, even the Buddha-nature and pure Mind (the moon). They have yet to realize that the conditioned self-conscious intellect (the mud puddle) is a deluded state of mind beholding an illusory copy of the real.