Knowledge of Katha Upanishad

His form is not to be seen. No one beholds Him with the eye. By controlling the mind by the intellect and by incessant meditation He is revealed. Those who know this (Brahman) become immortal.

When the five organs of knowledge are at rest together with the mind, and when the intellect ceases functioning (becomes calm), that they call the highest state.

Beyond the Avyakta is Purusha, all-pervading and devoid of Linga (indicative sign). He who knows Him is liberated and obtains Immortality.

Beyond the senses is the mind, higher than the mind is the intellect, higher than the intellect is the great Atman, higher than the Mahat is Avyaktam (the unmanifested).

A wise man, having understood that the senses, separately produced, are distinct from the Atman, and also their rising and setting, grieves no more.
Katha Upanishad