JUST as the Kathopanishad belongs to the Krishna (Black) Yajurveda, even so the Taittiriya Upanishad also belongs to the same Veda. Both these Upanishads have internal connection. In particular, the knowledge of the soul, which we find in the Kathopanishad, is confirmed and elucidated in the Taittiriya Upanishad. And it is to this aspect that we may turn at once.
The Kathopanishad speaks of three fires and five fires, indicating that the soul that is fire is to be found at three levels, the level of the body, the level of the vital being and the level of the mental being. The Kathopanishad also tells us that the mental being is not the highest, since the faculty of knowledge is even higher than the mind. It further tells us that there is even a greater self higher than the faculty of knowledge. Both these higher levels are accessible to the ascending fire of the soul, and when the soul ascends to these two, the fire of the soul does no more remain three-fold but becomes five-old.