There is no problem in the Ajahn Chah quote because he is not referring to some supposed metaphysical "original purity" that got defiled at a later stage. Instead, he is just reflecting on the present moment and how, when there is ignorance in the mind, mental fabrications are created which lead (following the sequence of dependent arising) to craving and grasping -- and how this whole structure can be disbanded when there is clear seeing and mindfulness watching over our consciousness.
There is a problem Bhante.
If we cannot accept the original mind, which is by nature pure, we also cannot accept another translation, which say this mind is naturally Peaceful.
We are back to the square. One issue is this mind is naturally pure, another one is naturally peaceful.
Different language, but same meaning. There is something which is naturally peaceful.
I can understand why some people do not accept something original pure. The reason is because they fail to differentiate unstained and defile. To them, these two terms are same.
If we see a mirror, we put the photo of bandit, the reflection of bandit appear.
If we replace that photo of bandit with photo of Buddha, the reflection of Buddha appear.
The mirror can be defiled by the appearance of bandit and Buddha. But, there is no way the photo of bandit can stain the mirror.
If the photo of bandit can stain the mirror, when you replace the photo of bandit with photo of Buddha, what will appear is photo of bandit. In this case, the mirror is stained by defilement of bandit. But reality is not like that. In reality, photo of bandit can defile the mirror, but there is no way photo of bandit can stain the mirror. Same thing photo of Buddha also cannot stain the photo I bandit.
Another example is the sky.
Today the sky can have white cloud, tomorrow dark cloud. The sky can be defiled by the appearance of white and dark cloud, but white and dark cloud cannot stain the sky.
If white cloud can stain the sky, after the appearance of white cloud, the dark white will never have a chance to appear. Similarly, if the dark cloud can stain the sky, after the appearance of dark cloud, white cannot appear anymore, because the sky has been damaged by the dark cloud. It has been stained.
Reality is not like that, the sky can be covered by cloud, but the sky can never be stained by cloud.
Another example is lotus.
Lotus grow in the mud. The mud can defiled the lotus, but it can never ever stain the lotus. Lotus is defileable by mud, but lotus is unstainable by mud.
Now if we go to the mind. If my mind can be stained by ignoranced, I will have no hope to become a Buddha anymore. This mind can be covered by thoughts, but this mind cannot be stained by thoughts. My previous thought cannot damage the luminosity of my mind. It also cannot improve the luminosity of my mind.
If we see the blue mountain, the appearance of blue mountain will appear in this mind.
But, even we become a Buddha, it is exactly same blue mountain will appear.
There is no way, this luminosity of mind can improve, so when we see the blue mountain, now blue getting more
The luminosity of mind is unstainable. Bad thought cannot stain this mind. Good thought cannot stand this mind. It can defile the mind, but it cannot stain the mind.
The nature of the sky is unstainable, but defileable. Similarly, the nature of mind is unstainable, but defileable.
If something is not naturally pure, no matter how hard we remove the defilement, it has no use.