Mental Hindrances Block all Understanding!

The Blessed Buddha once said:
There are five obstructing obstacles and hindrances of the mind, which
weakens knowledge. What five?
The mental hindrance of Sense-Desire is an overgrowth that blocks insight.
The mental hindrance of Ill-Will is a fire-like inner mental stumbling block.
The mental hindrance of Lethargy & Laziness is a ever delaying foggy prison.
The mental hindrance of Restlessness & Regret is like a stirring disturbance.
The mental hindrance of Doubt & Uncertainty is a wall-like high embankment.
Without having overcome these Five Mental Hindrances, it is impossible
for any Bhikkhu, whose understanding thus lacks both strength and power,
to understand, what is to his own advantage, to other's advantage, or to the
advantage of both. That he should be capable of attaining any supra-human
state: Noble Vision and Wisdom, that is indeed completely impossible...
But if a Bhikkhu has overcome these Five Mental Hindrances, these five
obstructing impediments, these overgrowths of the mind that dampen and
diminish any understanding, then it is very likely that he will understand
what is to his own advantage, another's advantage, & the advantage of both. 
Thus unhindered is he quite capable of attaining any supra-human state: And
thus Noble Vision and Wisdom...
It is like a fast flowing canal, where a man opens up 5 draining side-channels,
which then weaken, and divert the strong midstream. Similarly is indeed this
understanding stream of insight weakened and diluted in anyone who has not
yet overcome these Five Mental Hindrances, which block real understanding.
However, if the man closes down all the 5 draining side-channels, one by one,
then the midstream becomes stronger, and can carry anything with it!
Similarly with one unhindered by these 5 mental hindrances, his now freely
flowing understanding stream of insight is neither weakened, nor polluted,
nor diluted in any way, so he can easily recognize what is good for himself,
others and both. That such unhindered friend should be capable of the state
of Noble supra-human Vision and Wisdom, that is indeed quite possible...

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/The_Fire_of_Sense-Desire.htm
http://What-Buddha-Said.net/drops/II/Starving_the_Hindrances.htm
http://What-Buddha-Said.net/drops/II/Restlessness_and_Regret.htm
http://What-Buddha-Said.net/drops/II/Feeding_the_Hindrances.htm
http://What-Buddha-Said.net/drops/II/Doubt_and_Uncertainty.htm
http://What-Buddha-Said.net/drops/II/Lethargy_and_Laziness.htm
http://What-Buddha-Said.net/drops/IV/Suitable_Substitution.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/How_5_Become_10.htm
http://What-Buddha-Said.net/drops/II/Naturally_Radiant.htm
http://What-Buddha-Said.net/Canon/Sutta/AN/AN.I.3-4c.htm
http://What-Buddha-Said.net/Canon/Sutta/AN/AN.I.3-4.htm

Source (edited extract):
The Numerical Sayings of the Buddha. Anguttara Nikāya.
The Book of Fives 51: Unhindered... [III: 63-4]