Prajnanam Brahma — The Fourth Mahavakya
The Mahavakya 'Prajnanam Brahma' from the Aitareya Upanishad identifies pure consciousness/intelligence with Brahman — establishing that the ultimate nature of reality is awareness itself.
Prajnanam Brahma ('Consciousness is Brahman' — Aitareya Upanishad 3.3) completes the four Mahavakyas, each assigned to one of the four Vedas. Together, the four Mahavakyas constitute a complete teaching of non-dual recognition: Tat Tvam Asi ('That thou art' — the teacher's declaration to the student), Aham Brahmasmi ('I am Brahman' — the student's first-person recognition), Ayam Atma Brahma ('This Atman is Brahman' — the universal declaration), and Prajnanam Brahma ('Consciousness is Brahman' — the philosophical definition). Prajnanam (from prajna — pure awareness, intelligence) identifies the quality that makes consciousness what it is — its knowing, self-aware nature — with the ultimate reality. This is the most philosophically precise of the Mahavakyas: not just 'the self is Brahman' (which could be misunderstood as a particular self) but 'consciousness itself — the knowing capacity — IS Brahman.'
Prajnanam Brahma — Pure consciousness is Brahman.
Philosopher Gottfried Leibniz's monadology — which posited that the ultimate constituents of reality are mind-like simple substances (monads) — moves toward but does not reach the Advaita identification of consciousness with ultimate reality: Leibniz's monads are individuals, while Brahman is the one consciousness in which all apparent individuals appear.
Aitareya Upanishad