In plain words
But for men to live by the authority of these Vedas, some of its secrets must be recognised. It has described the whole essence of this Yajurveda in a few words alone. Within this Yajurveda, in its closing śloka, it set out the Īśāvāsya Upaniṣad, under the name of the Upaniṣads. The very first words are Īśāvāsyam idaṃ sarvam. Because it came with those very words, they named it the Īśāvāsya Upaniṣad. Above all the Upaniṣads, the Īśāvāsya Upaniṣad is the most exalted. It is sacred. The essence of all the Upaniṣads is in the Īśāvāsya Upaniṣad. What is its inner meaning? The whole world is one single Śakti. The whole world is one Ātma. What is in every jīva is God Himself. It has declared that the world holds no place where He is not, and no name that is not His. That is precisely why whoever hurts anyone hurts God. Whoever pleases anyone pleases God. They recognised this truth. In this, above all, he used to recite this śloka without ceasing. That is precisely why he took up and kept the vow, "Ahiṃsā is the highest Dharma; one must harm no one."
What is Ahiṃsā? Is it to be without harming anyone? Is that possible? It is impossible. Even in the small tasks we do in life we do many kinds of harm. In the steps we walk, so many living things go on dying. As we sweep our house, so many ants die. As we bathe, so many worms die. As we cook, so many germs in it are burnt to ashes. We need not even go as far as our tasks. In the very breaths we take in and let out, some lakhs of living things die in every single moment. So what is the true nature of this Ahiṃsā? Well, they said that Ahiṃsā is to be without harming anyone knowingly, by mind, word and body. Unknowingly we may do many things. For this we are not answerable. That is precisely why Tyāgarāja said, "O mind, knowingly think on Rāma." One must act knowingly; what is done unknowingly bears no fruit at all. Its pains do not touch us in the least.
We must not commit sin knowingly. And this doing is not only doing with the hands. We must not even think it in the mind. Not only must we not think it in the mind, we must not utter it with the mouth either. To purify mind, word and body, and to be without harming others, hurting them or wounding them: this is the right meaning of the word Ahiṃsā.
These Upaniṣads and the Vedas have gone on developing so many kinds of activity in order to proclaim the truth that God is all-pervading, the inner Ātma of all beings. What use is there in reciting any number of mantras? You must drink in the essence that is in the mantra.