Ishavasya is the Upanishad belonging to Shukla Yajurveda comprising 18 verses. It appears as the 40th adhyaya in Samhita. Ishavasya speaks about Karma, Vidya, Avidya and finally the truth that has to be realised.
What is the best and wisest way to live life? What is its ultimate meaning and goal? These are the questions people have been asking since the beginning of civilization. Sages and wise men have given several religions and ethical tests for our guidance. The Indian tradition gives us that philosophy known as Vedanta, the outcome of contemplations and meditations of several sages, who devoted their time for pursuit of truth. The sages had no intention of establishing an organized religion as we see now. Their objective was to examine the human being himself, who is the examiner and explorer of truth.
Examination of external world gives us the knowledge systems such as physics, chemistry or biology but does not give us knowledge of the examiner himself. The uniqueness of Indian sages is that their examination started with the human mind and its relation with Consciousness or Intelligence as we normally call. They explored the human society too, and explored the best way to live, while keeping our eye on the Supreme Reality.
Isavasya Upanishad is a small Upanishad which does not go into ontological complexities as the Mandukya Upanishad does, but goes into the question of ideal human life. The origins of the concepts of karma yoga, medications and bhakti yoga, which we see later in the Bhagavad Gita, can be found here. It also tells about the nature of Supreme Reality.