Amit Chaudhuri at the TLS:
The Upanishads, then, can hardly be called originary. They sound more like the latest in a series of disagreements; a great deal has preceded them, and reached a state of ossification before their arrival. Among what they challenge is a particular sense of causality regarding the relationship between creation and creator, which seems to have been extant when they were composed. Many traditions believe in a first cause, after which the universe comes into existence and before which there was nothing. The Upanishad’s conception of consciousness – “He moves, and he moves not”; “He is far, and he is near” – complicates the point of origin. Again, unlike Descartes’s belief that thought is both a product and a proof of existence, the Upanishad’s “What cannot be thought with the mind, but that whereby the mind can think” introduces an absence at the heart of thought. If thought can’t conceive whatever it is that produces it, then thought can’t be wholly present – a formulation that’s antithetical to the Cartesian proclamation. And since causality constantly reasserts itself as a default mode of thinking throughout history, the Upanishads remain, essentially, oppositional. They can’t occupy the space of established thought, being opposed to that space. Nor can one reduce either the Upanishads or the Gita in sociological terms to being “Brahminical” without losing sight of the fact that their language is critical-poetic – that is, they raise a critique through paradox and metaphor – rather than dogmatic or hieratic.
more here.