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