Swami Vivekananda on the Upanishads

The Upanishads have this one theme before them: “What is that knowing which we know everything else?” In modern language, the theme of the Upanishads is to find an ultimate unity of things. Knowledge is nothing but finding unity in the midst of diversity. Every science is based upon this; all human knowledge is based upon the finding of unity in the midst of diversity; and if it is the task of small fragments of human knowledge, which we call our sciences, to find unity in the midst of a few different phenomena, the task becomes stupendous when the theme before us is to find unity in the midst of this marvellously diversified universe, where prevail unnumbered differences in name and form, in matter and spirit—each thought differing from every other thought, each form differing from every other form. Yet, to harmonise these many planes and unending Lokas, in the midst of this infinite variety to find unity, is the theme of the Upanishads. … God is first taught as some one who is the Creator of this universe, its Preserver, and unto whom everything goes at last. He is one to be worshipped, the Ruler, the Guide of nature, external and internal, yet appearing as if He were outside of nature and external. One step further, and we find the same teacher teaching that this God is not outside of nature, but immanent in nature. And at last both ideas are discarded, and whatever is real is He; there is no difference. “Tat tvam asi, Śvetaketo—Shvetaketu, That thou art.” That Immanent One is at last declared to be the same that is in the human soul.

Complete Works of Swami Vivekananda