The human mind, in its constant attempt to classify, has tried to define the Upanishads as philosophy, or as a religious scripture. In the West, philosophy is mainly seen as something dry and dusty, based on intellectual attempts to create logical structures for a framework about the nature and purpose of existence. Anyone who has studied the great Western philosophers will recognise the type of mental gymnastics that goes by the name of philosophy. Religious scripture for the most part has tended to be dogmatic and fixated on justifying a particular way of relating to God and the universe. In Faust, Goethe decries the stifling and dead systems of philosophy and religion, and shows a path of redemption through experience and growth for his protagonist. It is, in the end, spiritual experience that reveals Truth to the seeker, not intellectual, logical deduction or inference.
The Upanishads defy these types of neat categorizations as they are not speculative philosophy nor credal religious tomes. In the Taittiriya Upanishad, Bhrigu approaches his father Varuna and asks him to “teach me the Eternal”. The teaching that follows us not an intellectual exercise or exposition, but a simple declaration to undertake concentration of thought on the following lines: ‘Seek thou to know that from which these creatures are born, whereby being born they live and to which they go hence and enter again; for that is the Eternal.’ The youth must then discover within himself the truth of existence.
Sri Aurobindo notes: “The Upanishads are the supreme work of the Indian mind. … The Upanishads are at once profound religious scriptures, — for they are a record of the deepest spiritual experiences, — documents of revelatory and intuitive philosophy of an inexhaustible light, power and largeness and, whether written in verse or cadenced prose, spiritual poems of an absolute, an unfailing inspiration inevitable in phrase, wonderful in rhythm and expression. It is the expression of a mind in which philosophy and religion and poetry are made one, because this religion does not end with a cult nor is limited to a religio-ethical aspiration, but rises to an infinite discovery of God, of Self, of our highest and whole reality of spirit and being and speaks out of an ecstasy of luminous knowledge and an ecstasy of moved and fulfilled experience, this philosophy is not an abstract intellectual speculation about Truth or a structure of the logical intelligence, but Truth seen, felt, lived, held by the inmost mind and soul in the joy of utterance of an assured discovery and possession, and this poetry is the work of the aesthetic mind lifted up beyond its ordinary field to express the wonder and beauty of the rarest spiritual self-vision and the profoundest illumined truth of self and God and universe. Here the intuitive mind and intimate psychological experience of the Vedic seers passes into a supreme culmination in which the Spirit, as is said in a phrase of the Katha Upanishad, discloses its own very body, reveals the very word of its self-expression and discovers to the mind the vibration of rhythms which repeating themselves within in the spiritual hearing seems to build up the soul and set it satisfied and complete on the heights of self-knowledge.”
Sri Aurobindo, The Upanishads, Introduction, page 1