'Soul Secret' in brief:

Know your SOUL (Atma) as riding in a chariot,
The BODY as the chariot,
Know thou the INTELLECT (Buddhi) as a chariot-driver,
And the MIND (manas) as the rein.

The SENSES (Indriya), they say, are the horses,
The OBJECTS OF SENSE, what they range over,
The SELF combined with SENSES and MIND…
Wise-men call it the ENJOY-ER ( Bhoktri).

Higher than the SENSES are the OBJECTS OF SENSE,
Higher than the OBJECTS OF SENSES is the MIND (Manas),
And higher than MIND is INTELLECT (Buddhi),
Higher than INTELLECT is the SELF (Atma).

-- Katha Upanishad