In plain words
Whatever we do, whatever we see, whatever we say, whatever we think, we must try to think of it in a sacred way. So in human nature we cannot take a step forward into any work without the resolve of the mind. The power of the mind's resolves is the root cause of the happiness and joy of the human race.
Man recognises the strength of his arms, the strength of his intellect, the strength of his mind, the strength of wealth and the strength of power, and simply sinks into folly. But we do not try to recognise down which path the great demon called Ahaṅkāra, lying asleep behind all of these, may push us. 'Śatamānaṃ bhavati śatāyuṣmān….': so people receive the blessing that man has a life of a hundred years. But what we see before us in daily life is that many beings die in the very middle of life. No one can settle what the truth of this is.
They spoke of a measure of a hundred years, but do not trust that word in your heart. In childhood, or in youth, or in old age, or in the last years, in the village, in the forest or in the midst of the waters, there is no knowing when it will be left.
So no one has the authority to settle this. But what is the inner meaning of saying a hundred years? All the śāstras have said that man has a hundred years. These are not merely descriptions made by learned men. This is the supreme truth. Every man must surely live a hundred years. It must be so. But what are the reasons for leaving midway? Ahaṅkāra, envy, bad feelings, bad conduct, arrogance: such knives, arising in man, cut man's span of life into pieces. These bad qualities alone keep bringing man's span of life to an end. The span of life is a full hundred years. Midway, evil forces of this kind enter and end this span of life. That is why only when man has good qualities, good conduct and good thoughts, such good qualities, does man's span of life fill out to a hundred years. This alone is the inner meaning of all the great ṛṣis of old living a hundred years.