What are the three doshas or faults in the mind?
They are: mala or impurities such as lust, anger and greed, vikshepa, tossing of the mind or mental oscillation, and avarana or the veil of ignorance. Imagine that there is a muddy lake covered with moss and the wind is blowing hard. The lake is the mind. The muddy condition represents mala. The agitation of the waters that is set up by the wind corresponds to the vikshepa in the mind set up by the vibration of prana. The moss that covers the surface of the water represents avarana, the veil of ignorance.
How can one make the mind subtle and pure?
Do japa. Do selfless service. Pray to God from the bottom of your heart. Have satsang. Meditate. Read the Bhagavad Gita and the Upanishads. Live alone. Live in seclusion for six months. Take sattwic food.
Does the mental equilibrium of an advanced yogi get affected when attacked by some serious disease?
No, it does not get affected. If there is any thought of the body, the disease, the bodily affliction or something that cannot be tolerated by the fleshly frame, remember that he is no advanced yogi, saint or sannyasin. He who has no thought of himself, the surroundings or the world, he who is centred in his own Self, his beloved ishta devata or gracious guru, and he who is entirely oblivious of limitations of any sort, identifying himself with the limitless, diseaseless, all-pervasive Brahman, is a true and advanced yogi, bhakta or jnani, and not otherwise. He can have no match in the whole world in the matter of utter indifference either towards his disease, towards his limited, perishable body or the whole world. He always remains in his own Self and never loses his balance under any circumstances. He firmly believes that he is the Infinite, the Absolute Brahman. He firmly believes that death awaits all and that the six urmis or evils: ashanaya (hunger), shoka (grief), moha (delusion), pipasa (thirst), jara (old age) and mrityu (death), are but common to all jivas, individual souls, and that he is the deathless Brahman. Hence there can be no mental upset for him even amid crucial tests.
What are the methods to develop unruffled serenity and mental composure under all conditions of life? Is it possible to attain absolute serenity?
Yes, by all means. Consider yourself as dead to the world or the world as dead to you. Develop atma vichara shakti, the power to contemplate on the Self. Identify not with the mind, senses and intellect or with any of the other modifications of the mind. Always engage yourself in thoughts of the Divine. Have no thought of yourself or the surrounding world. Be absolutely indifferent to yourself, as you ought to be towards the surroundings, the various daily happenings and the worldly repercussions. Knowledge of the Self, when once attained, absolves one of all mental disruptions and psychic derangements. Where there is firm-grounded knowledge of the Self in the spiritual seeker, where there is the consciousness in him that all indeed is Brahman, Sarvam khalvidam Brahma, and that he is none other than that Supreme Brahman, and where he understands that all happenings good, bad or mixed are but passing phases on the screen of the world, there can never be any imbalanced mental life.
It is the mind that is the sole cause for bondage. The mind is the substratum behind pleasure and pain, happiness and misery, success and defeat. Rise above the pairs of opposites by resorting to a guru and his instructions. Study the lives of saints who underwent various trials and the books by realized souls. Develop the spirit of true surrender wherein you have no thought of body or bodily needs or self-protection even in the slightest degree and wherein you forget the idea of life and death altogether. Serenity is merely mental. Therefore, cultivate mental equipoise by gaining spiritual knowledge.