Brahman as the Ground of Logic
Shankara's argument that Brahman/Atman is the presupposition of all logic, language, and inquiry — it cannot be doubted because doubt itself requires it — establishes the absolute primacy of consciousness in all philosophical investigation.
Every act of inquiry, doubt, or philosophical argument presupposes the existence of the inquirer/doubter/arguer — who is, in Advaita, Atman/Brahman. This means that no philosophical system can coherently deny the existence of consciousness, because the denial itself is a conscious act. Shankara argued this against the Buddhist philosophers (Madhyamikas and Vijnanavadins) who denied the existence of a permanent self: if there is no self, who is it that claims there is no self? The argument is not merely verbal — it points to the practical impossibility of performing a conscious act (including philosophical denial of consciousness) without the prior reality of consciousness. This argument anticipates not only Descartes' cogito but also the transcendental arguments of Kant (who identified the necessary conditions of the possibility of experience) and the phenomenological method of Husserl (who returned to 'the things themselves' — including consciousness itself — as the foundation of all knowledge).
Brahman cannot be doubted — for the doubter is Brahman. All denial of Brahman is self-defeating.
Philosopher Thomas Nagel's 'The View from Nowhere' — arguing that consciousness cannot be reduced to any third-person description because it is irreducibly first-personal — makes essentially the same point as Shankara: consciousness is the one thing that cannot be viewed from outside because it is the viewer of everything.
Brahmasutra Bhashya
Written about in
- What Advaita Vedanta says about doubt
He pressed the same point against the Buddhist philosophers who denied a permanent self: if there is no self, who is advancing the claim? Note the limit honestly — the argument secures consciousness, not the whole of Advaita's metaphysics, and the debate did not end with Shankara.