There are thirteen principal Upaniṣads—to understand their teachings one should begin with the four Mahavakyas (the Great Sayings):
1. Prajnanam Brahma (प्रज्ञानम् ब्रह्म) — “Pure Consciousness is Brahman" or "Brahman is insight” (Aitareya Upaniṣad, verse 3.3)
2. Tat Tvam Asi (तत् त्वम् असि) — “You are that” or “You are the existent” (Chāndogya Upaniṣad, verse 6.8.7)
3. Ayam Atma Brahma (अयम् आत्मा ब्रह्म) — "This Self (Atman or soul) is Brahman" (Māṇḍūkya Upaniṣad, verse 1.2)
4. Aham Brahma Asmi (अहम् ब्रह्मास्मि) - "I am Brahman" (Bṛhadāraṇyaka Upaniṣad, verse 1.4.10)
The traditional way of teaching the essence of the Upaniṣads to new students is through these four Mahavakyas.