Selfishness clouds understanding.

Even if a man has a tinge of selfishness, he cannot find out what is right and wrong. A very pure, subtle sharp intellect is needed for this purpose, besides destroying selfishness.

You should possess a well-purified heart. You must have a balanced mind. You must be indifferent to heat and cold, pleasure and pain, praise and censure, success and failure. You must be well-trained in selfless service for several years. Then you can get over selfishness and acquire real understanding.

~Swami Sivananda