In plain words
Selfishness grows. Sacrifice dwindles. Humanness itself is fading from sight. What is the reason? Losing faith in the Ātma. It is forgetting the principle that the power present everywhere is the power of the Lord of all. Antar bahiś ca tat sarvam vyāpya nārāyaṇa sthita: man makes no fitting effort to understand the Divinity that pervades within him, outside him, all around him, everywhere and at all times.
In the world there are rich people. There are strong people. There are people of good qualities. There are learned people. But people rich in the Ātma are nowhere to be seen. What does "rich in the Ātma" mean? The body is seen. The mind is felt to be there. The one who makes the unseen Ātma seen, and, absorbed and enraptured in that Ānanda, experiences Divinity, he alone is rich in the Ātma. In ancient times many great ṛṣis, rich in the Ātma like this, experienced this divine feeling, this divine essence, this divine power, rejoiced in it, and went on giving it to the world. To teach this supreme principle to ordinary man, they gathered together the ideas in the many Upaniṣads and went on proclaiming them to the world in the form of a triangle. One: the body. Second: the mind. Third: the Ātma. The body is the gross. The Ātma is the subtle. What harmonises the body and the Ātma, gross and subtle, joins the two, brings them together and makes them rejoice, is the mind. They showed that the setting forth of the harmony of these three is tattva. Man does not rightly understand the truth of this tattva. Tat-tvam. Tat means that. Tvam means this. What is it that we must recognise as "that"?
What is far from us we call THAT. What is near we call THIS. Far from what? Near to what? What is far from our senses they called That. What is near to our senses they called This. What is near to the senses is the body itself. And the tat that is far from the senses is the Ātma. The nature of the mind is to join, asi, the Ātma that is far with the body that is near. This is what they came to call Tat tvam asi. So we must try to recognise, in our humanness, the united form of these three: the body, the mind and the Ātma.