The 5 Impurities block the Lucidity of Mind:

The Blessed Buddha once said:
There are five impurities of gold impaired by which it becomes neither pliant
nor wieldy, it lacks radiance, is weak & easily broken and cannot be formed...
What are these five impurities? They are: Iron, copper, tin, lead and silver.
But if the gold has been purified from these five impurities, then it will indeed
be pliant and wieldy, radiant and firm, and can be formed well. Whatever kind
of jewellery one wishes to make from it, be it a diadem, earrings, a necklace or
a golden chain, it will easily serve that purpose well.
Similarly, there are five impurities of the mind impaired by which the mind is
neither pliant nor wieldy, it lacks radiant lucidity and stability, and cannot
concentrate well upon the eradication of the mental fermentations (āsava).
What are these five impurities? They are:

Sense-Desire,
Evil-Will,
Lethargy & Laziness,
Restlessness & Regret,
Doubt & Uncertainty...

But if the mind is freed of these five mental hindrances, then it will be plastic,
flexible, and wieldy, will be of radiant lucidity and firm calm stability, and will
concentrate well upon the elimination of the mental fermentations. Whatever
supra-human state realizable by these higher mental abilities one may pursue,
one will in each case be able to directly experience it, as an eye-witness...

Mind is naturally radiant!

 but veiled by mental hindrance!

Hindrances

More on  these 5 Mental Hindrances (Nīvaranas):
http://What-Buddha-Said.net/drops/II/The_5_Mental_Hindrances.htm
http://What-Buddha-Said.net/drops/II/Feeding_the_Hindrances.htm
http://What-Buddha-Said.net/drops/II/Starving_the_Hindrances.htm
http://What-Buddha-Said.net/drops/II/How_5_Become_10.htm
http://What-Buddha-Said.net/drops/II/The_Canal.htm
http://What-Buddha-Said.net/drops/II/Naturally_Radiant.htm
http://What-Buddha-Said.net/drops/IV/Suitable_Substitution.htm
http://What-Buddha-Said.net/Canon/Sutta/AN/AN.I.3-4.htm
http://What-Buddha-Said.net/Canon/Sutta/AN/AN.I.3-4c.htm
http://What-Buddha-Said.net/drops/II/The_Fire_of_Sense-Desire.htm
http://What-Buddha-Said.net/drops/II/Colourful_but_Muddy.htm
http://What-Buddha-Said.net/drops/II/Anger_and_Irritation.htm
http://What-Buddha-Said.net/drops/II/Lethargy_and_Laziness.htm
http://What-Buddha-Said.net/drops/II/Restlessness_and_Regret.htm
http://What-Buddha-Said.net/drops/II/Doubt_and_Uncertainty.htm
http://What-Buddha-Said.net/drops/II/The_Causes_of_Ignorance.htm

Source (edited extract):
The Numerical Sayings of the Buddha. Anguttara Nikaya.
The Book of Fives 23: Four deeds of Merit... [III: 16-7]

Comments:
The 4 mental fermentations are wrong, false & hidden assumptions associated with:
1: Sense-desire (kāmāsava). Ex: "Sensing is only & always pleasant. Pain exists not!"
2: Desiring becoming into new existence (bhavāsava): Ex: "All life is good, Death exists not!"
3: Wrong views (ditthāsava): Ex: "I am better, know better & what I think is thus never wrong!"
4: Ignorance (avijjāsava): Ex: "Suffering, craving, ending craving & the Noble Way exists not!"

Have a nice day!

  
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Naturally Radiant!