Nobody says scripture can stand alone as proof. Obviously, even Vedantins do not believe that scripture is proof by itself, hence why they use reasoning to justify what the scripture says and refute what is against scripture.
A statement like “the earth is flat” can be easily contradicted. There is no reasoning one could produce to justify this statement except that appears that the Earth is flat from an optical point of view. This can be contradicted from a 3D point of view.
However, the statement, “The Earth is round” can also be contradicted in the same way. It only appears the Earth is round from a 3D point of view, but actually there is no form as such from the atomic point of view. It is 99% empty space, and its solidity is owing to the electrons circulating rapidly around the nucleus producing the sensation of touch.
Vedanta considers this kind of reality which can be contradicted as relative and practical. It does not say it is unreal and not important, it says that it can ultimately be contradicted. Vedanta looks at the reality which cannot be contradicted and what cannot be contradicted is absolutely true. Now consider the postulations of Samkhya:
- Matter is changing, composite, active, objective, productive, has properties of pain and pleasure etc
- Consciousness is unchanging, non-composite, witnessing seeing, devoid of pain, pleasure and agency
Therefore
- The truth of matter can be contradicted
- The truth of consciousness cannot be contradicted
Consciousness simply is, it spaceless, timeless, absolute, free of all pain and pleasure. It is there even before there was manifest matter. It will be there even after there is no unmanifest matter. This truth is also accepted by Samkhya.
Now we can derive that if consciousness is spacless, timeless, absolute. It is identical to beingness and existence: Hence in Vedanta it is called sat(existence) and chit(consciousness). If consciousness is existence, then it means everything which happens happens within consciousness.
So Vedanta derive yet another conclusion: All forms(matter) are arising from consciousness and merging back into consciousness alone. Like all waves rise and merge back into the sea. Hence the Vedanta mahavakyas:
- All is consciousness
- You are consciousness
- I am consciousness
These truths can be supported by statements that cannot be contradicted(unlike the Earth is flat)
- If there is no consciousness, there is no reality
- Everything that happens is what we are conscious ‘of’ (i.e. is an object of our consciousnesses)
- The same consciousness that exists in the the state of deep sleep and dream, also exists in our waking sate