The Age of the Vedas and the principal Upanishads was the Age of Intuition, but this Age was followed by the Age of Reason. Inspired texts of the Veda and the Upanishads made room for metaphysical philosophy, even as afterwards metaphysical philosophy had to give place to experimental Science. The study of the history of the metaphysical philosophy of India demonstrates the great heights to which the pure reason developed, and the study of the experimental Science that developed in India demonstrates multisided development of the mixed action of the reason in minute subtlety and complexity; this mixed action of the reason explored the domains of experimental and pragmatic knowledge, and this afforded extreme possibilities of the development of the experiences of the physical mind and senses. This process can be seen as a circle of progress, since the results of the Age of Intuition came to be critically examined and assimilated by the Age of Pure Reason, and similarly the results of metaphysical philosophy came to be critically tested by the experiences and experiments that were meant to meet the demands of the mixed action of Reason and the physical mind and senses. In retrospect, it can be said that this succession and this attempt to separate assimilation enlarged the scope of inquiry and prevented the exclusive domination of any particular part of human consciousness and nature. A more complete harmony of different parts of knowledge was prepared.
In the development of philosophy that we see in the post-Upanishadic period, two distinctive stages can be discerned. In the first stage, Indian philosophers recognized the earlier results of Intuition as an authority superior to Reason. But at the same time, they started from Reason and tested the results it gave them, holding only those conclusions to be valid which were supported by the supreme authority of intuitive experience. They proceeded with the united consent of the two great authorities, Reason and Intuition. But in the second stage, the natural trend of Reason to assert its own supremacy triumphed. This explains the rise of conflicting schools even among those which founded themselves in theory on the Veda but used its text as a weapon against each other. This only illustrates how Reason functions; it proceeds by analysis and division and assembles its facts to form a whole; but in the assemblage so formed there are opposite anomalies, logical incompatibilities. In order to form a flawlessly logical system, Reason tends to affirm some aspects and to negate others which conflict with its chosen conclusions. The synthesis of intuitional knowledge that was present in the Veda and the early Vedanta was thus broken up, and devices, methods and standards of varying value were employed by which entire freedom was acquired for metaphysical speculations conducted by Reason.
Nevertheless, efforts were made from time to time to recombine philosophical systems into some image of the old catholicity and unity. Three great declarations of the Upanishads have constantly remained prominent during the course of the development of Indian philosophy, namely, "I am He", "Thou art That, O Shvetaketu", "All this is the
Brahman; this Self is the Brahman." The conceptions of Brahman, Purusha and Ishwara have remained alive, and they still carry something of the old burden of the inexpressible Reality. The questions which have always occupied the thought of India were concerning the relationship of the movement of becoming to any discoverable or realizable absolute Unity, and how the ego, whether generated by the movement or cause of the movement, can return to the true Self, Divinity, or Reality that was declared by the Vedanta.