In plain words
Impure water, impure foodstuffs, impure air: everything is impure through and through. Along with purifying our food and water, we must keep the mind pure too.
The Mind and Its Secret
The mind has become polluted. Man is wasting away through unrest of the mind alone, through ill health of the mind alone. People today think that diseases come to them as some kind of infection. A great mistake. Man has no disease at all. But imagining, psychologically, that he has a disease, he turns into a patient. 90% of man's diseases come about psychologically. It is diseases of the mind that come most of all. Worrying and worrying without end, he falls ill. There was one Galtin, a specialist in heart disease (heart doctor). Constantly examining other people's hearts, he would keep thinking, “How is my own heart, I wonder?” In the end he died of heart disease itself. Another doctor was Bleedy. He carried on research of every kind into digestion. Each time he examined a person's stomach, he would compare himself with it: “How is my own digestion, I wonder?” In the end he got indigestion and died. The influence of the mind is very great. That is why it was said, “Manayeva manuṣyāṇāṃ kāraṇaṃ bandha mokṣayoḥ”, that is, for bondage and for Mokṣa, for both, the mind alone is the root cause. If you set this mind on a sacred path, man takes shape as nothing but sacred, through and through. The work he does, the way he looks at things: all of it will be nothing but good. This is a lock. This is a key. Put this into the lock and turn it to the left, and the lock shuts. Turn it to the right, and the lock opens. To lock and to unlock there is one and the same key; the difference lies only in the turning. In the same way, the heart itself is the lock and the mind itself is the key. Turn it towards God, and there is release; turn it towards the world, and there is bondage. The mind alone is the cause of this bondage and of Mokṣa. Today we must do sādhana so that, instead of setting the mind in the world through material feelings, we turn it towards God.