The phrase "as the only valid" is correct and usable in written English.
It can be used when emphasizing that something is the sole acceptable or legitimate option in a given context.
Example: "The results of the experiment were conclusive, establishing the new theory as the only valid explanation for the observed phenomena."
Alternatives: "as the sole legitimate" or "as the exclusive valid".
Akalaṅka clearly has in mind a typical materialist philosopher who rejects the idea of omniscience, but at the same time accepts perception (pratyakṣa) as the only valid instrument of knowledge, while rejecting the validity of inference (anumāna).
Yet we regard elections as the only valid method.
Many see that ruinous path as the only valid expression of the referendum result.
Long touted by denialists as the only valid accounting, the list was never meant to be final.
"Tolerance is regularly criticised, while anti-immigration sentiment is presented as the only valid opinion," the journalist said.
They insist on winning the argument (imposing their opinion as the only valid one) rather than winning the client.
I never tell people this publicly as they wrongly assume the only valid creative state is that of the cliche of the tubercular, tortured artist coughing up blood in his garret.
And when that's not the reason, people are like, Well, then what?! as if cheating is the only valid excuse in the world for splitting.
It isn't true – as some reports imply – that the only valid visitors are family or professionals on a narrow time slot.
If the market, as the doctrine insists, is the only valid determinant of how societies evolve, and the market is dominated by giant corporations, then what big business wants is what society gets.
They concluded that Paraceratherium, as the oldest name, was the only valid indricothere genus from the Oligocene, and contained four valid species, P. bugtiense, P. transouralicum, P. prohorovi, and P. orgosensis.
