1. The symbols in the Upanishad pass from psycho-physical upwards into psycho-spiritual knowledge. These are still valid for those who practise Yoga and want to rediscover the secrets of our psycho-physical and psycho-spiritual being.
2. Examples of the above kind are Ajathashatru’s explanation of sleep and dream or the passages of the Prashna Upanishad on the vital principle and its motions.
3. There is a passage from the Isha in which Surya the Sun-God is invoked as the godhead of knowledge and his rays dispersed here on the mental level are the shining diffusion of the thought mind and conceal his own supramental truth.