We have noticed that during the closing period of the Sa@mhitâ there were people who had risen to the conception of a single creator and controller of the universe, variously called Prajâpati, Vis'vakarman, Puru@sa, Brahma@naspati and Brahman. But this divine controller was yet only a deity. The search as to the nature of this deity began in the Upani@sads. Many visible objects of nature such as the sun or the wind on one hand and the various psychological functions in man were tried, but none could render satisfaction to the great ideal that had been aroused. The sages in the Upani@sad had already started with the idea that there was a supreme controller or essence presiding over man and the universe. But what was its nature? Could it be identified with any of the deities of Nature, was it a new deity or was it no deity at all? The Upani@sads present to us the history of this quest and the results that were achieved. When we look merely to this quest we find that we have not yet gone out of the Âra@nyaka ideas and of symbolic (_pratîka_) forms of worship. _Prâ@na_ (vital breath) was regarded as the most essential function for the life of man, and many anecdotes are related to show that it is superior to the other organs, such as the eye or ear, and that on it all other functions depend. This recognition of the superiority of prâ@na brings us to the meditations on prâ@na as Brahman as leading to the most beneficial results. So also we find that owing to the presence of the exalting characters of omnipresence and eternality _âkâs'a_ (space) is meditated upon as Brahman. So also manas and Âditya (sun) are meditated upon as Brahman. Again side by side with the visible material representation of Brahman as the pervading Vâyu, or the sun and the immaterial representation as âkâs'a, manas or prâ@na, we find also the various kinds of meditations as substitutes for actual sacrifice. Thus it is that there was an earnest quest after the discovery of Brahman. We find a stratum of thought 44 which shows that the sages were still blinded by the old ritualistic associations, and though meditation had taken the place of sacrifice yet this was hardly adequate for the highest attainment of Brahman. Next to the failure of the meditations we have to notice the history of the search after Brahman in which the sages sought to identify Brahman with the presiding deity of the sun, moon, lightning, ether, wind, fire, water, etc., and failed; for none of these could satisfy the ideal they cherished of Brahman. It is indeed needless here to multiply these examples, for they are tiresome not only in this summary treatment but in the original as well. They are of value only in this that they indicate how toilsome was the process by which the old ritualistic associations could be got rid of; what struggles and failures the sages had to undergo before they reached a knowledge of the true nature of Brahman. Unknowability of Brahman and the Negative Method.