In plain words
Just as the strength of the body wastes away without food, the strength of the mind keeps wasting away through the senses. So man must keep these thoughts in check. He must keep his resolves and his doubts in check as well. Only then will man's power of mind grow, very intensely and in a holy way. Mental power is most necessary for man today. It is from the mind that man has become man. When mental power wastes away, man's purity too wastes away. By giving the senses zeal and encouragement out of greed, mankind keeps falling into sorrows and unrest of many kinds.
When we enquire into the cause of sorrow, its cause is Ajñāna itself. What is the cause of Ajñāna? Ahaṅkāra is the cause. What is the cause of Ahaṅkāra? Attachment is the cause. What is the cause of attachment? The body! In the end, sorrow comes from the body alone. When man masters the senses that are in this body, there will never be what is called sorrow for him, whether on the worldly path or on the spiritual path. Sorrow is not natural to man. It belongs only to the form of a cause; it is not natural to man. If sorrow itself were natural, no one could do any enquiry into it at all. One small example. Sweetness is natural to jaggery. However many ways you try to root out its natural sweetness, this sweetness cannot go. If what is called sorrow were natural to man, there would have been no ways at all to remove this sorrow. Whatever sādhanas one did, sorrow would not be removed. It is unnatural. That is why there are many ways to remove it. Sorrow is removed by thinking of God and by the spiritual path itself. However much nourishing food you give a lazy horse, it only grows lazier still. The senses are like a lazy horse. When we keep supplying them with all the nourishing food they ask for and everything they like, these senses grow ever stronger. Grown strong, in the end they destroy man's humanness. When we set the mind on the path of Satya, then we can enjoy Ānanda that is lasting, in the future.
One small example. We must recognise what kind of path we should follow through the senses. Three kinds of sorrow pursue man.