In Vedantic framework, Upanishads are regarded as the instrument of knowledge which reveals to me that I am Brahman. In Indian philosophy it is called Anumana by seeing something we inferred something further. Inferential knowledge accounts for a vast amount of our knowledge. And there are other sources of knowledge in Vedanta and Advaita Vedanta. But the interest to us is the source of knowledge which is called Upanishads.