The sutras commonly compare the Self-Nature to a mirror. The Mind-mirror of the Buddhas is bright， empty and still; the Mind-mirror of sentient beings is dull and clouded. However， the intrinsic nature of both mirrors （brightness） is the same. If sentient beings can wipe their Mind-mirror completely clean， they will return to their Self-Nature， the nature of the Buddhas -- they will become Buddhas. This is the basis of gradual cultivation， as taught in most Buddhist schools.