If you cannot take to this practice, then engage yourself in the cultivation of knowledge. Better than knowledge, however, is meditation, and better than meditation is renunciation of the fruits of action, for by such renunciation one can attain peace of mind.
(Bhagavad-Gita--12:12---purport).

One can see that even if one has enough money and adequate arrangements for sense gratification, there is neither happiness nor peace of mind. That is not possible, because one is situated in the mode of passion. If one wants happiness at all, his money will not help him; he has to elevate himself to the mode of goodness by practicing Kṛṣṇa consciousness.
(Bhagavad-Gita--14:17---purport).

The dirty things in the heart of a conditioned soul are the root cause of all troubles for him. A conditioned soul is surrounded by the manifold miseries of material existence, but on account of his gross ignorance he is unable to remove the troubles due to dirty things in the heart, accumulated during the long prison life in the material world. He is actually meant to serve the will of the Supreme Lord, but on account of the dirty things in the heart, he likes to serve his concocted desires. These desires, instead of giving him any peace of mind, create new problems and thus bind him to the cycle of repeated birth and death.
(Srimad Bhagavatam--2:1:22--purport).

Whatever a person may be in the estimation of the social order of things, if a person tries to reciprocate a feeling of love towards the Supreme Personality of Godhead and is satisfied with the blessings of the Lord, he will at once feel the highest peace of mind for which he is hankering life after life. Peace of mind, or in other words the healthy state of mind, can be achieved only when the mind is situated in the transcendental loving service of the Lord.
(Srimad Bhagavatam---2:6:6---purport).

People in general are always anxious to have peace of mind or peace in the world, but they do not know how to achieve such a standard of peace in the world. Such peace in the world is obtainable by performances of sacrifice and by practice of austerity.
(Srimad Bhagavatam---2:6:28---purport).

If one wants peace of mind and tranquillity in society, he must accept the fact that the real enjoyer is the Supreme Personality of Godhead. The Lord is the proprietor of everything all over the universe, and He is the supreme friend of all living entities as well. By understanding this, people can become happy and peaceful individually and collectively.
(Srimad Bhagavatam---4:24:66---purport).

Everyone is searching after peace of mind. This is obtainable only when one is completely freed from the desire for material sense gratification and is engaged in the devotional service of the Lord.
(Srimad Bhagavatam---5:7:11---purport).

If the members of human society actually want peace of mind, tranquillity and friendly relations between men and nations, they must follow the Kṛṣṇa conscious system of religion, by which they can develop their dormant love for Kṛṣṇa, the Supreme Personality of Godhead. As soon as people do so, their minds will immediately be filled with peace and tranquillity.
(Nectar of instruction).

A peaceful mind is the basis for becoming truthful, clean, equipoised, self-controlled and tolerant. Thus by attaining knowledge and knowing its practical application in life, one becomes convinced about the Absolute Truth.
(Krsna Book).