What is clean water? It is so transparent that we can see the bottom through it. This way, a clear mind is present when there isn’t dark thought. Not dark thought. How does the mind remain pure? Not having vain thoughts, thinking preciously reality, and trying to learn from the reality, the mind doesn’t become dark. Isn’t the water clean, as long as something is prevented from entering the water? In the same way, always trying not to permit bad things to enter the mind and taking actions to prevent it, learning, and trying to attain enlightenment, the mind can remain clear. As for enlightenment, when we do some work and come to know things in the work, that is, coming to understand a principle in it, we can attain the enlightenment about the work. Therefore, if you keep trying to enlightenment about things in your life you can keep your mind pure.
For example, if there is some sediments at the bottom of water and we stir the water, the water becomes unclear. If the sediments can be separated form the water, it can be clean. Purify them – we must –, expel them, or divide them to disappear and we can obtain clean water. The same thing can happen to a mind.