No date for the origin of the Upanishads can be fixed, because the written text does not limit their antiquity. The word Sruti makes that clear to us. The teaching probably existed ages before it was set down in any written form. The text itself bears evidence of this, because not infrequently in a dialogue between teacher and disciple the teacher quotes from earlier Scriptures now unknown to us. As Professor Max Müller states in his lectures on the Vedanta Philosophy: "One feels certain that behind all these lightning-flashes of religious and philosophic thought there is a distant past, a dark background of which we shall never know the beginning." Some scholars place the Vedic period as far back as 4000 or 5000 B.C.; others from 2000 to 1400 B.C. But even the most conservative admit that it antedates, by several centuries at least, the Buddhistic period which begins in the sixth century B.C. The value of the Upanishads, however, does not rest upon their antiquity, but upon the vital message they contain for all times and all peoples. There is nothing peculiarly racial or local in them. The ennobling lessons of these Scriptures are as practical for the modern world as they were for the Indo-Aryans of the earliest Vedic age. Their teachings are summed up in two Maha-Vakyam or "great sayings":--Tat twam asi (That thou art) and Aham Brahmasmi (I am Brahman). This oneness of Soul and God lies at the very root of all Vedic thought, and it is this dominant ideal of the unity of all life and the oneness of Truth which makes the study of the Upanishads especially beneficial at the present moment. One of the most eminent of European Orientalists writes: "If we fix our attention upon it (this fundamental dogma of the Vedanta system) in its philosophical simplicity as the identity of God and the Soul, the Brahman and the Atman, it will be found to possess a significance reaching far beyond the Upanishads, their time and country; nay, we claim for it an inestimable value for the whole race of mankind. Whatever new and unwonted paths the philosophy of the future may strike out, this principle will remain permanently unshaken and from it no deviation can possibly take place. If ever a general solution is reached of the great riddle . . . the key can only be found where alone the secret of nature lies open to us from within, that is to say, in our innermost self. It was here that for the first time the original thinkers of the Upanishads, to their immortal honor, found it...."