Keno Upanishad.

What is the supreme spirit? By what relation to our experience shall we understand it? This is the subject of enquiry of this Upanishad.

It clarifies in the outset itself that the Supreme spirit or Brahman's nature can neither be comprehended by the senses nor by human reasoning.This is so because the subject, object and means are all identical.It is Brahman by which ,understanding itself functions!

Brahman is that by which the mind thinks.

It is not one of the concepts that can be conceived by the mind.

It is that which enables the eye to see, ear to hear and breath to move.

These functions themselves depend on Brahman.Therefore these senses and mind are unable to comprehend Brahman.

Do not mistake this body that needs looking after, for the soul.

Life is not the aggregate of functions of the body but a function of the highest spirit , in as much as not a thought or a breath or a glance is possible without the Supreme agent.